公式法：
1.x^4-4
=(x^2+2)(x^2-2)
2.2a^2-4a
=2a(a-2)
3.a^2-b^2-2b-1
=a^2-(b^2+2b+1)
=a^2-(b+1)^2
=(a+b+1)(a-b-1)
4.2x³-8x
=2x(x^2-4)
=2x(x+2)(x-2)
配方法
5.x^2y+2xy^2+y^3
=y(x^2+2xy+y^2)
=y(x+y)^2
6.27x^2+18x+3
=3(9x^2+6x+1)
=3(3x+1)^2
7.ax^2-4ax+4a
=a(x^2-4x+4)
=a(x-2)^2
8.x²y-2xy²+y³
=y(x^2-2xy+y^2)
=y(x-y)^2
十字相乘法
9.x^2-4x-5
=(x-5)(x+1)
10.x^2+x-6
=(x+3)(x-2)
11.x^2-7x+10
=(x-2)(x-5)
12.a²-a-20
=(a-5)(a+4)