1*2*3+2*3*4+.+n*(n+1)(n+2) =1/4[(1*2*3*4-0*1*2*3)+(2*3*4*5-1*2*3*4)+.+ n(n+1)(n+2)(n+3)-(n-1)n(n+1)(n+2)] =1/4*n(n+1)(n+2)(n+3)
