How do you simplify (a^2-9)/a^2 * (a^2-3a)/(a^2+a-12)?

Jul 19, 2015

You first factorise, and then see if you can cancel.

Explanation:

$= \frac{\left(a + 3\right) \left(a - 3\right)}{a \cdot a}$ $\cdot \frac{a \left(a - 3\right)}{\left(a - 3\right) \left(a + 4\right)}$

$= \frac{a \left(a + 3\right) \left(a - 3\right) \left(a - 3\right)}{a \cdot a \left(a - 3\right) \left(a + 4\right)}$

$= \frac{\cancel{a} \left(a + 3\right) \cancel{\left(a - 3\right)} \left(a - 3\right)}{\cancel{a} \cdot a \cancel{\left(a - 3\right)} \left(a + 4\right)}$

$= \frac{{a}^{2} - 9}{{a}^{2} + 4 a}$