2X²-5X-12 =(2x+3)(x-4) 十字相乘
7X²-19X-6=(7x+2)(x-3) 十字相乘
X²Y²+2XY-8=(xy-2)(xy+4) 十字相乘
X²+2XY-8Y²
=X²+2XY+Y²-9Y²
=(X+Y)²-9Y²
=(X+Y-3Y)(X+Y+3Y)
=(X-2Y)(X+4Y)请问X²Y²+2XY-8=(xy-2)(xy+4) 是怎么算的 还是不太懂