| $$ \begin{aligned}\frac{x^2-16}{x^2+5x-36}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{x+4}{x+9}\end{aligned} $$ | |
| ① | Simplify $ \dfrac{x^2-16}{x^2+5x-36} $ to $ \dfrac{x+4}{x+9} $. Factor both the denominator and the numerator, then cancel the common factor. $\color{blue}{x-4}$. $$ \begin{aligned} \frac{x^2-16}{x^2+5x-36} & =\frac{ \left( x+4 \right) \cdot \color{blue}{ \left( x-4 \right) }}{ \left( x+9 \right) \cdot \color{blue}{ \left( x-4 \right) }} = \\[1ex] &= \frac{x+4}{x+9} \end{aligned} $$ |