法1 (2a+b)(2a-3b)-3a(2a+b) =2a(2a-3b)+b(2a-3b)-6a²-3ab=4a²-6ab+2ab-3b²-6a²-3ab=-2a²-7ab-3b²=-(2a+b)(a+3b)x(x+y)(x-y)-x(x+y)²=x(x²-y²)-x(x²+2xy+y²...