化简
【2的n+4次方-2*(2的2n次方)】/【2*(2的n+3次方)】
=【2的n+4次方-(2的2n+1次方)】/(2的n+4次方)
=【2的n+4次方】/(2的n+4次方)-(2的2n+1次方)/(2的n+4次方)
=1-2的n-3次方