1x2+2x3+...+100x101=1/3x100x101x1021x2+2x3+....nx(n+1)=1/3nx(n+1)x(n+2)1x2x3+2x3x4+....+nx(n+1)x(n+2)=1/4n(n+1)(n+2)(n+3)
(1)=1/3*100*101*102=343400(2)=1/3*n*(n+1)*(n+2) (3)=1/4*n*(n+1)*(n+2)*(n+3)