(1/4)x--[(1/2)x--(3/4)y^2]+[--(7/4)x+(1/4)y^2]
=(1/4)x--(1/2)x+(3/4)y^2--(7/4)x+(1/4)y^2
=--2x+y^2
因为 x=2分?,y=?,
所以 解不下去了