Answer
$$\frac{3}{1-\sqrt{7}}=-\frac{1+\sqrt{7}}{2}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{3}{1-\sqrt{7}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{3}{1-\sqrt{7}}\frac{1+\sqrt{7}}{1+\sqrt{7}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{3+3\sqrt{7}}{1+\sqrt{7}-\sqrt{7}-7} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}}\frac{3+3\sqrt{7}}{-6} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle4}{\textcircled {4}} } }}}\frac{1+\sqrt{7}}{-2} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle5}{\textcircled {5}} } }}}-\frac{1+\sqrt{7}}{2}\end{aligned} $$ | |
| ① | Multiply the numerator and denominator by the conjugate of the denominator . $$\color{blue}{ 1 + \sqrt{7}} $$. |
| ② | Multiply in a numerator. $$ \color{blue}{ 3 } \cdot \left( 1 + \sqrt{7}\right) = \color{blue}{3} \cdot1+\color{blue}{3} \cdot \sqrt{7} = \\ = 3 + 3 \sqrt{7} $$ Simplify denominator. $$ \color{blue}{ \left( 1- \sqrt{7}\right) } \cdot \left( 1 + \sqrt{7}\right) = \color{blue}{1} \cdot1+\color{blue}{1} \cdot \sqrt{7}\color{blue}{- \sqrt{7}} \cdot1\color{blue}{- \sqrt{7}} \cdot \sqrt{7} = \\ = 1 + \sqrt{7}- \sqrt{7}-7 $$ |
| ③ | Simplify numerator and denominator |
| ④ | Divide both numerator and denominator by 3. |
| ⑤ | Place a negative sign in front of a fraction. |
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