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## Problem

You want to perform a linear curve fit using standard least-squares formulas instead of Excel's linear trendline.

## Solution

Use Excel's built-in formulas such as `COUNT`, `SUM`, `SUMSQ`, and `SUMPRODUCT` to make it easy to apply the standard least-squares formulas.

## Discussion

The standard equation for a straight line is:

The standard least-squares formulas used to determine the slope, m, and intercept, b, of the fit line are:

In these equations, n is the number of data points. Excel has several built-in functions that make it very easy to compute the various sums that appear in the least-squares equations. These functions include:

`COUNT`

This function counts the number of cells containing numbers in a range of cells.

`SUM`

This function adds all the numbers in a range of cells.

`SUMSQ`

This function returns the sum of squares of the numbers contained in a range of cells.

`SUMPRODUCT`

This function sums the products of entries in corresponding ranges of cells.

Let's reconsider the data used in the example in Recipe 8.1. Instead of using a chart trendline to determine the best-fit line, we'll use the least-squares equations and built-in ...

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