(sum(r) for r=1..n)^2 - (sum(r) for r=1..n-1)^2 = n^3
well, we know that
n
∑ r = n(n+1)/2
r=1
so, square that and we have
n^2(n+1)^2/4
Sum to n-1 instead of n, and we have
(n-1)^2 n^2/4
Subtract to get
n^2/4 ((n+1)^2 - (n-1)^2)
= n^2/4 (4n)
= n^3
Prove that (n(top)sigma r=1(bottom))^2-(n-1(top)sigma r=1(bottom))^2 =n^3 general formula for both notation is r
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