Question

计算1×4＋2×5＋3×6＋4×7..＋15×18

Answer 1

通项an=n(n+3)=n^2+3n

Sn=n(n+1)(2n+1)/6+3n(n+1)/2

=n(n+1)(2n+10)/6

=n(n+1)(n+5)/3

=15×16×20÷3

=1600