通式为x的立方数列求和

Question

通式为x的立方数列求和

Answer 1

[n(n+1)/2]^2
证明1^3+2^3+3^3+...+n^3=(1+2+3+...+n)^2=[n(n+1)/2]^2
n^4-(n-1)^4
=[n^2-(n-1)^2][n^2+(n-1)^2]
=(2n-1)(2n^2-2n+1)
=4n^3-6n^2+4n-1
2^4-1^4=4*2^3-6*2^2+4*2-1
3^4-2^4=4*3^3-6*3^2+4*3-1
4^4-3^4=4*4^3-6*4^2+4*4-1
.
n^4-(n-1)^4=4n^3-6n^2+4n-1
各等式全部相加
n^4-1^4=4*(2^3+3^3+...+n^3)-6*(2^2+3^2+...+n^2)+4(2+3+4+...+n)-(n-1)
n^4-1^4=4*(1^3+2^3+3^3+...+n^3)-6*(1^2+2^2+3^2+...+n^2)+4(1+2+3+4+...+n)-(n-1)-2
n^4-1=4*(1^3+2^3+3^3+...+n^3)-6*n(n+1)(2n+1)/6+4*n(n+1)/2-n-1
n^4-1=4*(1^3+2^3+3^3+...+n^3)-n(n+1)(2n+1)+2n(n+1)-n-1
n^4-1=4*(1^3+2^3+3^3+...+n^3)-n(n+1)(2n+1)+2n(n+1)-n-1
4*(1^3+2^3+3^3+...+n^3)
=n^4-1+n(n+1)(2n+1)-2n(n+1)+n+1
=n^4-1+(n+1)(2n^2-n)+n+1
=n^4-1+(2n^3+n^2-n)+n+1
=n^4+2n^3+n^2
=(n^2+n)^2
=(n(n+1))^2
1^3+2^3+3^3+...+n^3
=[n(n+1)/2]^2