49(x-y)的平方-25（x+y）的平方
=[7(x-y)]^2-[5(x+y)]^2
=[7x-7y+5x+5y][7x-7y-5x-5y]
=(12x-2y)(2x-12y)
=4(6x-y)(x-6y)
49(x-y)的平方-25（x+y）的平方 .
49(x-y)的平方-25（x+y）的平方
=[7(x-y)]^2-[5(x+y)]^2
=[7x-7y+5x+5y][7x-7y-5x-5y]
=(12x-2y)(2x-12y)
=4(6x-y)(x-6y)