设∑n·n=a n^3+b n^2+c n
n=1,a+b+c=1
n=2,8a+4b+2c=5
n=3,27a+9b+3c=14
{{a =1/3,b = 1/2,c = 1/6}}
设∑n·n=1/3 n^3+1/2 n^2+1/6 n=n(n+1)(2n+1)/6