x的2次方(a—1)+4(1—a)
=(x²-4)(a-1)
=(x+2)(x-2)(a-1)
(a+b)(a—b)—5(a的平方—2b的2次方)
=a²-b²-5a²+10b²
=9b²-4a²
=(3b+2a)(3b-2a)