y³+7y²-8 =y³-y²+8y²-8=y²(y-1)+8(y²-1)=y²(y-1)+8(y-1)(y+1) =(y-1)(y²+8y+8) 到这一歩就行了如果继续分解 =(y-1)(y-4+2√2)(y-4-2√2)