x为正数,y为负数所以x-y是正数,所以/x-y+1/=x-y+1
x为正数,y为负数所以-x是负数y-x-3是负数,所以/y-x-3/=x+3-y
x为正数,y为负数所以/y-x/=x-y
所以原式=x-y+1-2*（x+3-y)+x-y+5
=x-y+1-2x-6+2y+x-y+5
=0