1. Division Rule (Keep, Change, Flip)
$\frac{A}{B} \div \frac{C}{D} = \frac{A}{B} \cdot \frac{D}{C}$
2. Greatest Common Factor (GCF)
$ax \pm ay = a(x \pm y)$
3. Difference of Squares
$a^2 - b^2 = (a - b)(a + b)$
4. Perfect Square Trinomial
$a^2 \pm 2ab + b^2 = (a \pm b)^2$