ab应该不能为负数吧.
1=（a+b）²≥4ab
0≤ab≤1/4
设t=ab∈[0,1/4]
f(t)=t^n+t^(-n)在[0,1/4]上单调递减
f(t)≥f(1/4)=4^n+4^(-n)