·为什么1^3+2^3+........+n^3=(1+2+3........+n)^2

http://baike.baidu.com/view/1722042.html?wtp=tt

∑i^3
=∑(i+1)i(i-1)+∑i
=6∑C(3,i+1)+∑C(1,i)
=6C(4,n+2)+C(2,n+1)
=(n+2)(n+1)n(n-1)/4+(n+1)n/2
=[n(n+1)/2]^2

题目有没有错 啊 左边是3次的 右边才2次题目就错了怎么