Question
Answer
$$\dfrac{10}{7-2\sqrt{5}}=\dfrac{70+20\sqrt{5}}{29}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{10}{7-2\sqrt{5}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{10}{7-2\sqrt{5}}\frac{7+2\sqrt{5}}{7+2\sqrt{5}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{70+20\sqrt{5}}{49+14\sqrt{5}-14\sqrt{5}-20} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}\frac{70+20\sqrt{5}}{29}\end{aligned} $$ | |
| ① | Multiply the numerator and denominator by the conjugate of the denominator . $$\color{blue}{ 7 + 2 \sqrt{5}} $$. |
| ② | Multiply in a numerator. $$ \color{blue}{ 10 } \cdot \left( 7 + 2 \sqrt{5}\right) = \color{blue}{10} \cdot7+\color{blue}{10} \cdot 2 \sqrt{5} = \\ = 70 + 20 \sqrt{5} $$ 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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