(x-y)/(x+y)=4
所以(x+y)/(x-y)=1/4
(x-y)/[2(x+y)]-(4x+4y)/(x-y)
=(1/2)*[(x-y)/(x+y)]-4[(x+y)/(x-y)]
=(1/2)*4-4*(1/4)
=1