1.解:(1)x-y+3. (2)x+3+y. (3)-x+y+3. (4)3-x-y.
2.解:(1)错误.应为x²-(3x-2)=x²-3x+2.
(2)错误,应为7a+(5b-1)=7a+5b-1.
(3)正确.
(4)错误,应为-(a-b)+(ab-1)=-a+b+ab-1.
3.解:(1)原式=4ab-a²-b²+a²-b²-3ab = (-a²+a²)+(-b²-b²) +(4ab-3ab) =-2b²+ab.
(2)原式=x-1-x-4x+8=(x-x-4x)+(-1+8)=-4x+7.