# How do you find the sum of the series #i^2# from i=1 to 12?

##### 2 Answers

#### Answer:

#### Explanation:

We have the formula

#### Answer:

#### Explanation:

Here's a non-standard way to do it without having to remember individual formulae for different kinds of sequences...

The sequence of squares looks like this:

#1, 4, 9, 16, 25,...#

So the sequence of sums of squares from

#color(blue)(0), 1, 5, 14, 30, 55,...#

The sequence of differences of that sequence is the sequence of squares:

#color(blue)(1), 4, 9, 16, 25,...#

The sequence of differences of the sequence of squares is a linear (arithmetic) sequence:

#color(blue)(3), 5, 7, 9,...#

The sequence of differences of that sequence is a constant sequence:

#color(blue)(2), 2, 2,...#

We can then use the first term of each of these sequences as coefficients to give a formula for the sum of

#S_n = color(blue)(0)/(0!) + color(blue)(1)/(1!) n + color(blue)(3)/(2!) n(n-1) + color(blue)(2)/(3!) n(n-1)(n-2)#

#=n + 3/2n^2-3/2n+1/3n^3-n^2+2/3n#

#=1/6(2n^3+3n^2+n)#

#=1/6n(n+1)(2n+1)#

So:

#S_12 = 1/6*12*13*25 = 650#