a（x*2-2x）²-2ax（2-x）+a
=a[（x(x-2）)²-2x（2-x）+1]
=a[（x(x-2）)²+2x（x-2）+1]
=a（x(x-2）+1)²
=a（x^2-2x+1)²
=a（x-1)^4