[x²(x+y)²+2(x+y)²(x-y)-3(x+y)³]÷[- 1/2(x+y)²]
=-2(x+y)²[x²+2(x-y)-3(x+y)]÷(x+y)²
=-2(x²-x-5y)
化简至此,如果题目中已知x和y的值,不妨代入上式即得.