n^2 = n(n+1) -n =(1/3)[n(n+1)(n+2) -(n-1)n(n+1)] -(1/2)[n(n+1) -(n-1)n]1^2+2^2+...+n^2=(1/3)n(n+1)(n+2) -(1/2)n(n+1)=(1/6)n(n+1)( 2(n+2)-3)=(1/6)n(n+1)(2n+1)
