2(2的n+3)次方分之2的n+4次方-2(2的n次方)
=[2^(n+4)-2*2^n]/[2*2^(n+3)]
=[2^(n+4)-2^(n+1)]/2^(n+4)
=2^(n+4)/2^(n+4)-2^(n+1)/2^(n+4)
=1-2^(n+1-n-4)
=1-2^-3
=1-1/8
=7/8