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Factoring w/ Identities

Difference of Squares
a2−b2=(a−b)(a+b)a^2-b^2=\left(a-b\right)\left(a+b\right)

Difference of Cubes
a3−b3=(a−b)(a2+ab+b2)a^3-b^3=\left(a-b\right)\left(a^2+ab+b^2\right)

Sum of Cubes
a3+b3=(a+b)(a2−ab+b2)a^3+b^3=\left(a+b\right)\left(a^2-ab+b^2\right)

Example
Factor the following:

a) x2−4x^2-4
(x−2)(x+2)\left(x-2\right)\left(x+2\right)

b) x3−27x^3-27
(x−3)(x2+3x+9)\left(x-3\right)\left(x^2+3x+9\right)

c) (x+1)3+8\left(x+1\right)^3+8
((x+1)+2)((x+1)2−(x+1)(2)+4)\left(\left(x+1\right)+2\right)\left(\left(x+1\right)^2-\left(x+1\right)\left(2\right)+4\right)
=(x+3)(x2+2x+1−2x−2+4)=\left(x+3\right)\left(x^2+2x+1-2x-2+4\right)
=(x+3)(x2+3)=\left(x+3\right)\left(x^2+3\right)


d) x4−16x^4-16
This is a difference of squares (x2)2−(4)2\left(x^2\right)^2-\left(4\right)^2:
(x2−4)(x2+4)\left(x^2-4\right)\left(x^2+4\right)
=(x−2)(x+2)(x2+4)=\left(x-2\right)\left(x+2\right)\left(x^2+4\right)