答案：x-y=-15（x-y)^2-6(x-y)+9=-4xy[(x-y)-3]^2=-4xyx-y=3+2√（-xy)设-xy=z，带入上式得4z-6-10√z=0设√z=t,由上式可得4t^2-10t-6=0得到t=3,可得到xy=-9,所以：x-y=-15
