1^2=1/6*1（2*1+1）（1+1）=1/6*6=1
1^2+2^2=1/6*（2*2+1）（2+1）=1/6*30=5
.
假设1方+2方+3方+……+N方=1/6n（2n+1)(n+1)
则
1^2+2^2+3^2+……+n^2+（n+1）^2
=1/6n（2n+1)(n+1)+(n+1)^2
=1/6(n+1)(2n^2+n+6n+6)
=1/6*(n+1)(2n+3)(n+2)
=1/6*(n+1)[2(n+1)+1][(n+1)+1]
假设成立
得证