因为是因式分解题,所以中间符号不是乘,是减.
9(X+Y)^2-25(X-Y)^2
=［3(X+Y)+5(X-Y)］［3(X+Y)-5(X-Y)］
=(8X-2Y)(-2X+8Y)
=-4(4X-Y)(X-4Y).