它是个菱形这个我刚好前几天做过当时是用特殊值法做的其实，你可以分四种情况：x-y>=0,x+y>=0x-y>=0,x+y<=0x-y<=0,x+y>=0x-y<=0,x+y<=0分别把四条直线的方程求到
x>y 时:2(a+b)x+2(a-b)y=1x<=y时:2(a-b)x+2(a+b)y=1
