解
(x-2)(x-4)+1
=x²-6x+8+1
=x²-6x+9
=(x-3)²
ax²-2a²x+a³
=a(x²-2ax+a²)
=a(x-a)²
a²-2ab+b²-1
=(a-b)²-1
=(a-b+1)(a-b-1)