证明就是反复利用平方差公式：n=1+(n−1)⋅(n+1)−−−−−−−−−−−−−−−−−√f(x)=1+x1+(x+1)1+(x+2)1+...−−−−−√−−−−−−−−−−−−−−−−√−−−−−−−−−−−−−−−−−−−−−−−−−−−√−−−−−−−−−−−−−−−−−−−−−−−−−−−−−√−−−−−...