Answer
$$\frac{7}{5-\sqrt{5}}=\frac{35+7\sqrt{5}}{20}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{7}{5-\sqrt{5}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{7}{5-\sqrt{5}}\frac{5+\sqrt{5}}{5+\sqrt{5}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{35+7\sqrt{5}}{25+5\sqrt{5}-5\sqrt{5}-5} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}\frac{35+7\sqrt{5}}{20}\end{aligned} $$ | |
| ① | Multiply the numerator and denominator by the conjugate of the denominator . $$\color{blue}{ 5 + \sqrt{5}} $$. |
| ② | Multiply in a numerator. $$ \color{blue}{ 7 } \cdot \left( 5 + \sqrt{5}\right) = \color{blue}{7} \cdot5+\color{blue}{7} \cdot \sqrt{5} = \\ = 35 + 7 \sqrt{5} $$ Simplify denominator. $$ \color{blue}{ \left( 5- \sqrt{5}\right) } \cdot \left( 5 + \sqrt{5}\right) = \color{blue}{5} \cdot5+\color{blue}{5} \cdot \sqrt{5}\color{blue}{- \sqrt{5}} \cdot5\color{blue}{- \sqrt{5}} \cdot \sqrt{5} = \\ = 25 + 5 \sqrt{5}- 5 \sqrt{5}-5 $$ |
| ③ | Simplify numerator and denominator |
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