**=SUMXMY2(C2:C50,C1:C49)/SUMSQ(C1:C50)**

The SUMXMY2 function sums the squares of X minus Y. Notice that two ranges lie in the parentheses following this function. The first range (here C2:C50) lists the X values and the second range (here C1:C49) lists the Y functions. So the first difference is C2-C1, the second is C3-C2, and the last will be C50-C49. There will be 49 differences which are squared and summed.

The denominator employs the SUMSQ functions which squares the values in a range and then sums the squares. Here we square all 50 residuals, then sum the squares.