4*2^2n*2^(n-1)=2^2*2^2n*2^(n-1)=2^(3n+1)
4乘以2的2n次方乘以2的n-1次方=2的2次方乘以2的2n次方乘以2的n-1次方=2的2+2n+n-1次方=2的3n+1次方
4*2^(2n)*2^(n-1)=2^2*2^(2n)*2^(n-1)=2^(2+2n+n-1)=2^(3n+1)
