Question
Answer
$$\dfrac{-9}{6+\sqrt{15}}=\dfrac{-54+9\sqrt{15}}{21}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{-9}{6+\sqrt{15}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{-9}{6+\sqrt{15}}\frac{6-\sqrt{15}}{6-\sqrt{15}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{-54+9\sqrt{15}}{36-6\sqrt{15}+6\sqrt{15}-15} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}\frac{-54+9\sqrt{15}}{21}\end{aligned} $$ | |
| ① | Multiply the numerator and denominator by the conjugate of the denominator . $$\color{blue}{ 6- \sqrt{15}} $$. |
| ② | Multiply in a numerator. $$ \color{blue}{ -9 } \cdot \left( 6- \sqrt{15}\right) = \color{blue}{-9} \cdot6\color{blue}{-9} \cdot- \sqrt{15} = \\ = -54 + 9 \sqrt{15} $$ Simplify denominator. $$ \color{blue}{ \left( 6 + \sqrt{15}\right) } \cdot \left( 6- \sqrt{15}\right) = \color{blue}{6} \cdot6+\color{blue}{6} \cdot- \sqrt{15}+\color{blue}{ \sqrt{15}} \cdot6+\color{blue}{ \sqrt{15}} \cdot- \sqrt{15} = \\ = 36- 6 \sqrt{15} + 6 \sqrt{15}-15 $$ |
| ③ | Simplify numerator and denominator |
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