How Does Summation Notation Transition in This Example?

[LearninDaMath]

Homework Statement

What is happening from the green box to the blue box? I am completely lost as to what this example is trying to show. Would really appreciate some info on this, thanks.

Homework Equations

 

[LCKurtz]

This problem assumes you know the formula for the sum of squares of the first n integers. But the sum you are given isn't the over first n integers because it starts at 4. So the idea is to express the sum you want as the difference of two sums for which you know the formula.

 

[LearninDaMath]

How can I learn what this example assumes I already know? Is this covered in algebra textbooks?

 

[LCKurtz]

How can I learn what this example assumes I already know? Is this covered in algebra textbooks?

Yes, usually.