I don't understand the sigma notation and for loop

Question

I have the book saying that

and says that this is equivalent to saying

Answer 1

I think that we have limitations for calculating For loops through Sigma notation

for example

x=0
For i=2 to n-1 do
    For j=i+1 To n-2 do
          x=x+1

for n=5 we have x=1

but when I calculate it by Sigma

Sigma i=2 to n-1 (Sigma j=i+1 to n-2)= (n^2 -7*n+10 )/2

if we substitute n=5 then answer is zero, Why?