| $$ \begin{aligned}\frac{a^2+a-20}{a^2-9a+20}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{a+5}{a-5}\end{aligned} $$ | |
| ① | Simplify $ \dfrac{a^2+a-20}{a^2-9a+20} $ to $ \dfrac{a+5}{a-5} $. Factor both the denominator and the numerator, then cancel the common factor. $\color{blue}{a-4}$. $$ \begin{aligned} \frac{a^2+a-20}{a^2-9a+20} & =\frac{ \left( a+5 \right) \cdot \color{blue}{ \left( a-4 \right) }}{ \left( a-5 \right) \cdot \color{blue}{ \left( a-4 \right) }} = \\[1ex] &= \frac{a+5}{a-5} \end{aligned} $$ |