Question
Answer
$$\dfrac{-3}{7+2\sqrt{5}}=\dfrac{-21+6\sqrt{5}}{29}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{-3}{7+2\sqrt{5}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{-3}{7+2\sqrt{5}}\frac{7-2\sqrt{5}}{7-2\sqrt{5}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{-21+6\sqrt{5}}{49-14\sqrt{5}+14\sqrt{5}-20} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}\frac{-21+6\sqrt{5}}{29}\end{aligned} $$ | |
| ① | Multiply the numerator and denominator by the conjugate of the denominator . $$\color{blue}{ 7- 2 \sqrt{5}} $$. |
| ② | Multiply in a numerator. $$ \color{blue}{ -3 } \cdot \left( 7- 2 \sqrt{5}\right) = \color{blue}{-3} \cdot7\color{blue}{-3} \cdot- 2 \sqrt{5} = \\ = -21 + 6 \sqrt{5} $$ Simplify denominator. $$ \color{blue}{ \left( 7 + 2 \sqrt{5}\right) } \cdot \left( 7- 2 \sqrt{5}\right) = \color{blue}{7} \cdot7+\color{blue}{7} \cdot- 2 \sqrt{5}+\color{blue}{ 2 \sqrt{5}} \cdot7+\color{blue}{ 2 \sqrt{5}} \cdot- 2 \sqrt{5} = \\ = 49- 14 \sqrt{5} + 14 \sqrt{5}-20 $$ |
| ③ | Simplify numerator and denominator |
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