How to perform linear interpolation in Excel with the mathematic formula 1. Select the column of y-values in the same table and name those “yvalues”. Bessel’s Interpolation Formula For the derivation of Bessel’s formula, taking the Mean of the Gauss’s Forward formula and Third Gauss’s formula thus we achieve the Bessel’s Formula as 3. Step 2: Creating UDF with VBA. 1. Masukkan formula =E8+B3 di cell E9, Masukkan formula =E9+B3 di cell E10, Dan kita akan langsung mendapatkan hasilnya, Figure 8 Ini dia hasilnya. We can use the following basic syntax to perform linear interpolation. The weights are based on the distance between the corners and the point (x, y). Is it possible for excel to find missing data and to use the known y's either side of the cell to interpolate the results or the previous 2 know y's to extrapolate the resultsRe: 3 Variable Table Lookup with linear Interpolation. The first parameter, NewX is simply the value to interpolate. I have a simple chart in which the X axis is logarithmic and the Y axis is linear. In the dataset, there is information on the weekly sales of a shop. How does Excel compute its smoothing algorithm for curved lines in the XY scatter plots?Interpolation. Basic usage of interpolate(). As we can see from the image above, point P is situated in the rectangle defined by the values 22. For example, with GRIDXYZ you can easily interpolate scattered (x,y,z) data points onto a uniform rectilinear grid for easy surface plotting in Excel. Returns the one-dimensional piecewise linear interpolant to a function with given discrete data points ( xp, fp ), evaluated at x. If we want to mean interpolate the missing values, we need to do this in two steps. ADVERTISEMENT. , (xn, yn) is defined as the concatenation of linear interpolants between each pair of data points. One-dimensional linear interpolation for monotonically increasing sample points. STEP 4: Using VLOOKUP to Find Next Year’s Cash Flow. 674), for 40 C in the first column. Interpolate Excel Formula Linear Interpolation – This method is quick and easy to use, also known as. 01 Y2 = 5. Step 3. Step 3: Finally, the interpolated point will be. Afterwards, we fill the NaNs with interpolated values by calling the interpolate() method on the read value column:The interpolation function will always be continuous, but may not be differentiable: Neat Examples. y = y 1 + (x-x 1)(y 2-y 1)/(x 2-x 1). xll. 1 x = 0. Have a look at Fig. One might even use the exponential interpolation for loss payouts espoused in Berquist and Sherman (1977). Indeed, SPSS and Minitab use the interpolation formula shown. 子供と楽しく貼ることができました。購入当時、鳥は貼らない予定でしたが、子供が大喜びで、鳥さんここに貼る♪と本当はいけないのでしょうが、何度も貼り直しては遊んでいました（笑）シールもストレスなく台紙から剥がせました。Option Explicit Function Linterp2 (rX As Range, rY As Range, x As Double) As Double ' linear interpolator / extrapolator ' R is a two-column range containing known x, known y Dim lR As Long, l1 As Long, l2 As Long Dim nR As Long 'If x = 1. Here is the formula in action in a worksheet:Interpolate metric is probably the most mysterious one since nothing in SuccessFactors highlights the mathematical formula leading to the value we see in Incentive Payout % when we open the worksheet. Step 1: Navigate to the File menu and click on Options, the last thing on that list. If you right-click on this trendline, you can adjust its settings and display properties of it, including the equation used to draw it. Step 2: Click on the “Insert” tab on the uppermost ribbon in the Microsoft Excel sheet and select the “Recommended Charts” option. The x-coordinates at which to evaluate the interpolated values. XlXtrFun has been used for years by engineering and research and development personnel on every. In this blog, we. In this article, we discuss how you can interpolate missing data in Excel with elaborate explanations. Introduction; Formula; Interpolation example; Extrapolation example; Calculator; Interpolation in embedded systems; Introduction. Interpolation Formula Excel. Select one of the "2-D Line" thumbnails that the drop-down box displays. Equation. Step 2: Input Required Data into Cubic Spline Model. On a log-log plot, the same formula can be used by simply replacing all y and x values with their logarithms. Count If XRates. INC will interpolate to determine the value at the k-th percentile. Next, click on the bottom right corner of cell B2 and drag the formula down to fill in the remaining cells in column B. 5. Method 2: Interpolate in Excel Graph Using Trendline. In the window that pops up, click Regression. In our example, the time taken changes by 1 minute when the speed of the bike. Key words: interpolate, interpolation, graph, data set points, INTERPXYThis video demonstrates how to use the function INTERPXY() to interpolate on a graph g. After entering all of the required values, the linear interpolation calculator automatically generates for you the values of m and b given the formula slope intercept form: y = mx + b. How to Do Bilinear Interpolation in Excel. Methods. interpolate with. 00% And I want to interpolate the y-values for x-vales 2-5,7-11, and 13-17. This calculator allows calculating kinematic viscosity at 40°C from the known VI and kinematic viscosity of the oil at 100°C: The common equation used to calculate the viscosity by interpolation between two reference points is with the Ubbelohde-Walther equation, which is adopted in ASTM D341. In summary, linear interpolation can be used to find y for a known x on a linear scale plot, using the formula y = y1 + (x-x1) (y2-y1)/ (x2-x1). 260%. In such a case, instead of the MINIFS function you'll have the MATCH and INDEX functions to find the value in the row that follows. This won't follow the nice curves along the XY plot, but it might still suit your purposes. The instance of this class defines a __call__ method and. Rem Linear Interpolation from a x and y ordered data, for a value: Rem TODO: make it work with unordered Data. Excel allows me to linearly interpolate between two values using Fill>Series. interpolate (method='linear') Try it Yourself ». 3. Also please find here an Excel. Description. A percentile calculated with . XLookup is a great replacement for index and match. It’s simple & easy, just follow along. See the screenshot below. You can also use Excel to calculate a regression with a formula that uses an exponent for x different from 1, e. I want to do this by hand without using any python libraries. Excel will plot your function's logistic growth on a chart. Plug this information into the. The column index is provided by the MATCH function. The formula works by: using the MATCH function to find the row in the inputs range just before the value being searched for (e. Step 1: Finding the Value of Z. To calculate monthly interest rate, the formula in C6 is: =RATE (C2*12, C3, ,C4) Please note that C2 contains the number of years. 3 is the value just before 3. S. HTH, Bernie MS Excel MVP Function Interpolate(Xnow As Double, _ XRates As Range, YRates As Range) As Double Application. In numerical analysis, polynomial interpolation is the interpolation of a given bivariate data set by the polynomial of lowest possible degree that passes through the points of the dataset. PERCENTILE function examples. Method 1: Mathematical Equation for Linear Interpolation. In conditional formatting options, selects color scales. ETS function syntax has the following arguments: Target_date Required. I have a function I found that will perform a linear interpolation. Method 3: Interpolate in Graph Using SLOPE and INTERCEPT Functions. You will use the FORECAST function when you need a function to forecast your data without making charts and graphs. This the ‘Bezier’ function. Jerry. Implementing Weighted Moving Average Formula. Conclusion. Here is a simple calculator to interpolate. known_x’s – a range of x-coordinates. 75%. Here we can apply the Lagrange’s interpolation formula to get our solution. You can monitor the calculated values in the Visual Basic Immediate window. If the values in column B do not increase in relation to the values in column A, then the MINIFS function will not return the correct value. You can use the limit argument to specify the maximum number of consecutive NaN values to interpolate. STEP 5: Computing Fractional Year Value with ABS Function. Row or column: axis Maximum number of consecutive NaN to fill: limit; Direction to interpolate: limit_direction Interpolate or extrapolate or both: limit_area Operate inplace: inplace Interpolation method: method Linear interpolation: linear, index, values Using existing values: ffill, pad, bfill, backfill. Basically, I have 4 numbers which represent the y-values for an 18-set of numbers. Assuming an Alpha value of 0. Multiple regression equation: y = b 1 x 1 + b 2 x 2 +. So E8 will lie between 3 and 4, based on the. So if you have 2-d table, you can find out which formula fits best your data, using Excel's "trend line" feature. Then, we will go to the name box to the left of the formula bar and input “ xvalues ”. Let’s look at some examples of how to interpolate in. 2. The formula coincides with the standard Lagrange interpolation formula if the points are given in R2. For example, Excel spits out a different value than Mintab or SPSS do. Choose the Scatter chart type and select the scatterplot. Then we can use one of the many websites about interpolate formulas in mathematics. xlsm (with macro). By the way, our spreadsheet can interpolate on log-log and log-lin axes. 3 is the value just before 3. Linear Interpolation Formula Excel. Perform Interpolation of Linear 2D Data in Excel. The TREND function returns values along a linear trend. 67 for every 1 of X. 1. HOW TO ATTACH YOUR SAMPLE WORKBOOK: Unregistered, Fast answers need clear examples. x 1. Example 1: Point Estimate for a Population Mean. Description. Know the formula for the linear interpolation process. , "eyeballed"). One of: ‘linear’: Ignore the index and treat the values as equally spaced. For n n data points, the unknowns are the. As an example, if you want to find the saturated pressure of water at a temperature of 40 C you can look in Table B. If you need to, you can adjust the column widths to see all the data. Select the range of 12 data points for the X Function Argument field. import pandas as pd. Double Interpolation is very familiar to Engineers who are using it for every Design ProblemsThose who want to do Double Interpolate in excel this Video will. Volatile Dim hi As Long Dim lo As Long Count = XRates. I would not worry about the volatile nature of the OFFSET formula if you do not have too many. The =INDEX and =MATCH pick out the relevant two points on known_y's and known_x's. x0 : a 1d-array of floats to interpolate at x : a 1-D array of floats sorted in increasing order y : A 1-D array of floats. The basic idea is the same as I usually propose for linear interpolation problems, but the step where you find the slope and intercept are done in log-log space instead of linear space: 1) The lookup table in H2:J9 where I use the LINEST () function to compute slope and intercept for each pair of points. Close the VBA window and use the newly created function by typing =InterpolateTable (). The formula to calculate linear interpolation is: Linear Interpolation (y) = y1+(x−x1) (y2 −y1) (x2 −x1) y 1 + ( x − x 1) ( y 2 − y 1) ( x 2 − x 1) where, x1 x 1 and y1 y 1 are the first coordinates. From the iteration calculations so far, we know that at the lower discount rate rL of 8. Interpolate Missing Data in Excel: 4 Easy Ways. 0 7. 1 Answer. We can add another series to highlight the input data points. Percentile IF in table. To create a heat map in Excel you need to follow the below steps: First of all, select the data on which you want to apply a heat map (here you need to select all the cells where you have sales values) After that, go to Home Tab Styles Conditional Formatting. ) and some manually. The. Every time you change one of the referenced values the calculation will be run again. These are useful for interpolation. Once we have our data set, we must create a chart based on those values. If our data set follows a linear pattern, we can use this method to. In this case, F7=7 and E7=3 because B already included the integer 7 and has a matching value in column A. EXC function returns the k-th percentile of values in a range, where k is. Cubic Spline Interpolation in Excel: Step-by-Step Procedure. Effective Interest Rate = (1 + i/n)n – 1. 0 NaN # 3 3. Step 4: Create 3D Interpolation Graph. Surprisingly, linear interpolation is quite hard to do in Excel. We will have to interpolate the in between yields for different periods, such as for 3-year, 3-year, 6-year, and so on. Contents. Interpolated Yield Curve - I Curve: A yield curve derived by using on-the-run treasuries. Jon Peltier says. This tutorial provides a step-by-step example of how to fit an equation to a curve in Excel. You need four points instead of two in order to get the curviness right – this is also how you know it’s a cubic spline. Follow the steps below to enable Solver under Excel. I chose the interval [0, 5] [ 0, 5] and if n n denoted the number of equally spaced intervals into which [0, 5] [ 0, 5] was divided, I applied Newton-Forward interpolation formula for various values of n n from 1 to 100 and recorded the corresponding approximate values of f f at x = 0. 674), for 40 C in the first column. In other words, SMALL can return the "nth smallest" value (1st smallest value, 2nd smallest value, 3rd smallest value, etc. In the Insert menu, click on the Scatter icon and click on the scatter chart option with smooth lines and markers. Step 3: Prepare Dataset According to X and Y value. The N-1 construction makes sense to me, if you think of the fencepost analogy. radial basis function. I was using the excel 'fill' function to fill the blank cells as a linear trend but the full dataset spans 15 years; I found this process quite slow since I could only fill 2/3 months at a time. Step 3: Prepare Dataset According to X and Y value. I want it to be an exponential function so it's. There's no logarithmic regression or interpolation in Excel, even in the Anlaysis ToolPak. Linear Interpolation formula is a method that constructs the new data points from the given set of data points. For example, if we know the past earnings and expenses, we can forecast the future. The formula used for interpolation uses the slope formula and the point-slope form for a linear equation. For Non-Integer Month number, I need use some kind of "linear interpolation" approach to get an approximate LMS data. First, the value of x will go, then add the y-axis values and finally add x-axis values in the formula and click Enter. Replace NULL values with the number between the previous and next row: In this example we use a . Call the value that comes before B1. An extrapolated expression is an expression used to estimate the value of a dependent variable for an independent variable that is said to be certainly outside the scope of a particular known dataset and to compute a linear search. Count If nR < 2 Then Exit Function If x < rX (1) Then ' x < xmin, extrapolate l1. Locate where 12 bar a (A) would be. FORECAST. An instance of this class is created by passing the 1-D vectors comprising the data. Step 2: Creating UDF with VBA. Given two known values (x 1, y 1) and (x 2, y 2), we can estimate the y-value for some point x by using the following formula: y = y 1 + (x-x 1)(y 2-y 1)/(x 2-x 1) Here are the steps for performing linear interpolation in Excel using the mathematic formula: 1. You can use these functions to predict future sales, inventory requirements, or consumer trends. Method 2: Interpolate in Excel Graph Using Trendline. 1 Linear Interpolation Property tables such as steam tables are tabulated at discrete values of the specific properties. - So, we've seen two ways to calculate in-betweens: linear interpolation and Bézier curves. 2242 percent and the 60-day interest rate. I have assumed values for December 2020, 2021, 2022 and 2023 and want to estimate the values for the month in between. From there, you can find the estimated value at any point between them. 3. Copy the code below into a code module, and use as your describe. Click Insert > Module, then paste below VBA code to the. The above use of LinearInterpolation is actually a short hand for a composition of interpolate, scale, and extrapolate. However, F8=8, which is an intermediate value not contained in column B. Interpolation is a common way to approximate functions. The different types of this concept include linear interpolation among others which are discussed below. I wish to linearly interpolate the quarterly data to monthly data so I can conduct some analysis between the two sets of financial data. Join Wallstreetmojo Instagram. In our example, Y moves by 1. 22% 18=100. Methods vary from linear to more theoretical. EXC function calculates the "kth percentile" for a set of data. 5 Then Stop nR = rX. Add-in capabilities. 968741* (25) + 2. 00% And I want to interpolate the y-values for x-vales 2-5,7-11, and 13-17. The more data points that are used in the interpolation, the higher the degree of the resulting polynomial, and therefore the greater oscillation it will exhibit between the data points. R esultTo do this, you start with known values, and you estimate the unknown points through interpolation. In this method, we will use a mathematical formula to calculate logarithmic interpolation in Excel. Type. 75 years] in the par term structure. To use an interpolation function, click on the fx button on the formula bar, select the XonGrid 4 interpolation category, select a function then enter your data. The prefix “inter” means “in. The continuity of first derivative means that the graph y = S(x) will notDescription. Free, download now. The calculator plots the interpolated point on a line and shows a step-by-step solution by using linear interpolation formula. 05734 – 2. Interpolate in Excel Graph: 6 Ways. Click on DATA along the top ribbon, then click the Data Analysis option on the far right. In other words, LARGE can retrieve the "nth largest" value. To execute a linear interpolation in Excel, we use the forecast function and the lookup function to create a straight line that connects two values and estimates the future value through it. The formula used by Excel to calculate slope is the same one used by the SLOPE function: In the example shown, the intercept formula can be manually created like this:Create a new sheet. Enter the MATCH function. Steps: First of all, let’s assume Depth of Flow = X and Discharge = Y. INTERPXY is a versatile 2D interpolation function based on splines. INC will interpolate when the value for the specified percentile is between two values in the array. Each method is quite easy. Create a chart with your information. The most common example of a sigmoid function is the logistic sigmoid function, which is calculated as: F (x) = 1 / (1 + e-x) To calculate the value of a sigmoid function for a given x value in Excel, we can use the following formula: =1/ (1+EXP (-A1. + b n x n + a. – 2. The formula of linear interpolation is given by-Linear Interpolation (y) = y 1 + [(x-x 1) × (y 2-y 1)]/ (x 2-x 1)This is a sample XLL (DLL for Microsoft Excel) that adds linear interpolation capability to the Excel function list. =IF(B2<>C2,([put formula here]),XLOOKUP(H2,A:A,B:C)) It's saying if B2 an C2 are different, then run your formula, otherwise return the two values in B and C from the lookup value set in H2. K is the rank you need to determine. This adjustment corrects calculations that yielded incorrect R 2 values and aligns the R 2 calculation with the LINEST function. Select the row of x-values in the data table and name those “xvalues”. See this paper for a very nice discussion of these algorithms. To do this, we can simply select the row containing the x-values. To interpolate linearly in Excel, follow these steps: Enter your data into an Excel worksheet. Utilizing FORECAST Function for Data Extrapolation in Excel. Excel® has a Graphic assistant but if you like export data to Autocad®, you can do the following procedure:The cubic spline interpolation is a piecewise continuous curve, passing through each of the values in the table. When you have only one independent x-variable, the calculations for m and b are based on the following. Interpolation is a mathematical function that takes the values of nearby points and uses them to predict the value of the unknown end. The linear interpolation formula requires that maturity be expressed in days, yet designated maturities are often expressed in months . INC function calculates the "kth percentile" for a set of data, where k is between 0 and 1, inclusive. Given three known data points (x₀, y₀), (x₁, y₁), and (x₂, y₂), each of which is composed of an x-value and its corresponding y-value, the quadratic interpolation method is used to obtain an estimated value, y, at a given x-value, x. =B2+ (E2-A2)* (B3-B2)/ (A3-A2)Step 1: Locate the x-value, i. In this section we will only consider polynomial interpolation, but other sorts of functions can be very useful as well, such as rational or trigonometric functions. 2. It requires two pieces of information: the array and the quart. For example, we have the formula y = 3x 2 - 12x + 9. Hi, I have lists with data for 3 different vehicles' speed vs grade %. Key Takeaways. Table of Contents hide. Step 2: A new window will pop up named Excel Options. i. Inclusion of this interpolation function to replace the VBA code cut down the calculation time to a matter of. Because on-the-run treasuries are limited to specific maturities, the yield of maturities that lies. Use INTERPXY to interpolate from a set of (x,y) data points at an arbitrary point. In such cases, interpolation is required to obtain the correct value. It can be used to probe the value of a surface passing through a set of scattred data points at arbitrary points. . ★ Want to automate Excel? Check out our training academy ★ Download the example file★ The syntax and usage of the two functions are the same. 5) using OFFSETs to select the square of that line and the next (in light purple) using FORECAST to build a linear interpolation using just those two points, and getting the resultLinear interpolation is useful when looking for a value between given data points. In other words, interpolate the column E values corresponding to the column F values. Then select the chart you want to extract data from and press Alt + F11 keys simultaneously, and a Microsoft Visual Basic for Applications window pops. The MATCH function is used to get a column index for VLOOKUP like this: MATCH(H5,Table1[#Headers],0) This is. That is, it is recalculated every time any cell in any worksheet in the workbook is edited, as well as after certain other operations. Cubic Spline Interpolation in Excel: Step-by-Step Procedure. , see screenshot: Step 3: In Fill Blank Cells dialog box, check the Linear values option and From left to right option. Polynomial. Interpolation is a technique for adding new data points within a range of a set of known data points. For x1 and y1, use the input and output from the lower value, and for x2 and y2, use the input and output from the higher value. FORECAST. If P(x) is evaluated at x = xk, all the products except the kth are zero. The X and Y data are in columns A and B. This will work on Excel 2007, but =FORECAST. Excel's solver tool is a really powerful way to perform nonlinear interpolation. Therefore, you cannot ask the interpolation of a single number - rather, you need multiple numbers to determine your line, and from there you can ask what value you would. To interpolate this value, use the following simple formula: (x – x1) * y= y1 + (x – x1) x=22. Interpolation technique to use. Right click on your chart > Add trendline > Under Trendline options, select one by one and see which is the best fit > Go down and check Display Equation on Chart. Extract the Q 11, Q 21, Q 12 and Q 22 points. The Microsoft Excel PERCENTRANK function returns the rank of a value in a set of values as a percentage of the set. Most common practise is to linearly interpolate. I'm trying to understand how to use linear interpolation with an x,y, and z plane. It's easy to calculate y for any given x. In other words, interpolate the column E values corresponding to the column F values. The values are not equidistant from each other. Here, we have a dataset containing the values of X and Y, where Y=F(X). Verma @ 1 person found this reply helpful. The length of y along. A function to linearly. Step3: After rearranging the equation, the value of a comes out to be,. Excel's LINEST function can also calculate multiple regressions, with different exponents on x at the same time, e. g. This can be done using the simple interpolation formula however it will not be very accurate. This is a small modification of Jacob Jan Tuinstra's script, enabling it to take either an array or a value as the third argument, so that interpolated function can be computed at many places at once. Then, insert two columns named LogX and LogY. If k is not a multiple of 1/(n + 1), PERCENTILE. Add input cells for height and span in J1 and J2 respectively. , yn corresponding to x = x0, x1 ,. INTERPXY automatically. Log-linear would be wrong; forward points are commonly negative, and are merely a delta on the Spot. Curve fitting, on the other hand, is a broader process of constructing a curve, or mathematical function, that. 2 Chapter 3. Excel for Microsoft 365 Excel for Microsoft 365 for Mac Excel for the web Excel 2021 Excel 2021 for Mac Excel 2019 Excel 2019 for Mac Excel 2016 Excel 2016 for Mac Excel 2013 Excel Web App Excel 2010 Excel for Mac 2011 Excel Starter 2010 More. You could do a linear interpolation, which I describe in a tutorial called Excel Interpolation Formulas on my blog. Step 2: Now click the button “Calculate” to get the result. Interpolación lineal en Excel: ejemplo paso a paso. This equation will help you for interpolation. Copy/paste the formula into other cells with missing values, (provided that you don't have 2 missing values in a row). But when the given X value or sigma for which I'm trying to interpolate is between the highest and lowest sigma I get incorrect results. Using Formula for Data Extrapolation in Excel. 5 in this case. Click on the image of the example steam table to open that image in a new window in a larger size to be easier to see it clearly. Then, click on the Insert Line or Area Chart drop-down on the Charts group. In more detail, the FORECAST function performs the actual interpolation using the linear interpolation equation shown above. Extrapolation is defined as an estimation of a value based on extending the known series or factors beyond the area that is certainly known. The corresponding desired pressure is then in the next column; in this case, 7. Google groups is your friend. You'll need much more advanced software for that, such as MatLab. To find the first x-value from the table of data, use the following formula: =XLOOKUP(F3,B5:B10,B5:B10,,-1,1) F3 is the lookup_value, and the array B5:B10 is both the lookup_array and the return_array. The approach, of course, is to. The domain of s is in intervals of [a, b]. 95(11+1)=11. In the interpolation example…. Can. Usually labeled Saturation Temperature and Saturation Pressure. 2: using the formula: =LINEST(B2:B21, A2:A21^1. Method-4: Do VLOOKUP and Interpolate in Excel Using the FORECAST, OFFSET, and MATCH Functions. g. Method-3: Using the Combination of the IF, ISNA, VLOOKUP, AVERAGE, INDEX, and MATCH Functions. When adding a trendline in Excel, you have 6 different options to choose from. xll is a collection of functions which extends the capabilities of Microsoft Excel; developed primarily to facilitate, interpolation of 2-dimensional and 3-dimensional data, and simplify 2-variable curve fitting. Actuaries are consistently faced with the decision of how to interpolate loss development factors. Jon Peltier says. I am trying to replicate what excel is doing with the percentile. To do this, click on Chart Design > Select Data. ‘index’, ‘values’: use the actual numerical values of the index. 1. Excel provides a QUARTILE function to calculate quartiles. Furthermore, the kth product is equal to one, so the sum is equal to yk and the interpolation conditions are satisfied. This tip explains how to use the function. print(df. Linear Interpolation in Excel: 7 Methods. The Excel PERCENTILE. Locate the temperature 40°C in the left hand column of the table. In the dataset, there is information on the weekly sales of a shop. Table of Contents hide. (What you see basically is a curve which is constituted of linear segments. Method-6: Using VBA Code to Do VLOOKUP and Interpolate in Excel. Excel inserts curly brackets at the beginning and end of the formula for you. Dear Excel friends, I want to linearly interpolate between multiple points. Applying FORECAST Function for Exponential Interpolation in Excel.