a²（x-y)+b²（y-x)
=(x-y)(a²-b²)
=(x-y)(a+b)(a-b)
2-½x²
=2(1- x²/4)
=2(1+½x)(1-½x)
（a+2b）²-4（a-b）²
=[(a+2b)+2(a-b)][(a+2b)-2(a-b)]
=(a+2b+2a-2b)(a+2b-2a+2b)
=3a(4b-a)
9（a+b)²-4（a-b）²
=[3(a+b)+2(a-b)][3(a+b)-2(a-b)]
=(3a+3b+2a-2b)(3a+3b-2a+2b)
=(5a+b)(a+5b)