设 nnn 是正整数，求证：
(1+2n)n>=1n+2n+4n+6n+...+(2n)n(1+2n)^n>=1^n+2^n+4^n+6^n+...+(2n)^n(1+2n)n>=1n+2n+4n+6n+...+(2n)n
