Question

完全平方差公式怎么解来的?这些公式都是怎么解得?1、(x-y)?-(x+y)?（或者全是+呢?）2、[x-y+3]?

Answer 1

(x-y)-(x+y) =(x-y+x+y)(x-y-x-y)=-4xy [x-y+3]=(x-y)^2+6(x-y)+9=x^2-2xy+y^2+6x-6y+9