问题描述:
1×2+2×3+3×4+…+n×(n+1)=?用n的式子表示
最佳答案:
1×2+2×3+3×4+……+n(n+1)=1/3n(n+1)(n+2)
解:1×2+2×3+3×4+…+n(n+1)
=(1^2+1)+(2^2+2)+(3^2+3)+…(n^2+n)
=(1^2+2^2+3^2+……+n^2)+(1+2+3+...+n)
而,1^2+2^2+3^2+……+n^2=n(n+1)(2n+1)/6
1+2+3+……+n=n(n+1)/2
则:1×2+2×3+3×4+……+n(n+1)
=(1^2+2^2+3^2+……+n^2)+(1+2+3+……+n)
=n(n+1)(2n+1)/6+n(n+1)/2
=1/3n(n+1)(n+2)
补充回答:
一点都老
网友(127.255.255.*)2019-11-04