（X³-2x²+x-4)-(2x³-5x-4) =x³-2x²+x-4-2x³+5x+4=x³-2x³-2x²-4+4=-x³-2x²3(a²-2a-3)-5(-5a²+a-2)=3a²-6a-9+25a²-5a+10=3a²+25a²-6a-5a-9+10=28a²-11a+1
括号打开，合并同类项，即可，手机不太方便过程，抱歉…
