2的n次方减2的负n次方=6,
2^n-2^(-n)=6
[2^n-2^(-n)]^2=36
2^2n-2*2^n*2^(-n)+2^(-2n)=36
2^2n-2+2^(-2n)=36
2^2n+2^(-2n)=38
4^n+4^(-n)=38