n(n+1)
=n^2+n
Sn=1x2/2+2x3/2+3x4/2+.+nx(n+1)/2
=1/2*[1x2+2x3+3x4+.+nx(n+1)]
=1/2*[1^2+1+2^2+2+3^2+3.+n^2+n]
=1/2*[1^2+1+2^2+2+3^2+3.+n^2+n]
=1/2*[n(n+1)(2n+1)/6+n(n+1)/2]
=1/12*[2n(n+1)(2n+1)+3n(n+1)]
=1/12*n(n+1)[2(2n+1)+3]
=1/12*n(n+1)(4n+5)
=n(n+1)(4n+5)/12