当n=k+1时
左边=(n+2)(n+3)……(n+n)（n+n+1)(n+1+n+1)=
=2^n×1×3×……×(2n-1)（n+n+1)(n+1+n+1)/(n+1)=
=2^n×1×3×……×(2n-1)（2n+1)×2
=2^(n+1)×1×3×……×(2n-1)（2n+1)=右边
从右面开始算更简单