Question

In Lecture Notes 18 on about the second page it is said:

1^2+2^2+...+n^2>1/3(base)(height)=1/3n^2*n

So all of this has to do with the area under x^2 so if b=10 and n=5 that means the area under x^2 is divided into 5 parts each part being 2 units wide. So for the b and n I specified what would be the actual value for (base) and the actual value for (height). This is not clear to me.