3（x+4）（x+3）-2（x-7）（x+2）=3(x²+7x+12)-2(x²-5x-14)=3x²+21x+36-2x²+10x+28=x²+31x+49（a+b－2c）（a-b+2c）=[a+(b-2c)]×[a-(b-2c)]=a²-(b-2c)²=a²-(b²-4bc+4c²)=a²-b²+4bc-4c²
速度你妹啊，这么简单还问
