Answer
$$\frac{3}{\sqrt{21}}=\frac{\sqrt{21}}{7}$$
Explanation
Tap the blue circles to see an explanation.
| $$ \begin{aligned}\frac{3}{\sqrt{21}}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}} \frac{ 3 }{\sqrt{ 21 }} \times \frac{ \color{orangered}{\sqrt{ 21 }} }{ \color{orangered}{\sqrt{ 21 }}} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle2}{\textcircled {2}} } }}}\frac{3\sqrt{21}}{21} \xlongequal{ } \\[1 em] & \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle3}{\textcircled {3}} } }}} \frac{ 3 \sqrt{ 21 } : \color{blue}{ 3 } }{ 21 : \color{blue}{ 3 } } \xlongequal{ } \\[1 em] & \xlongequal{ }\frac{\sqrt{21}}{7}\end{aligned} $$ | |
| ① | Multiply both top and bottom by $ \color{orangered}{ \sqrt{ 21 }}$. |
| ② | In denominator we have $ \sqrt{ 21 } \cdot \sqrt{ 21 } = 21 $. |
| ③ | Divide both the top and bottom numbers by $ \color{blue}{ 3 }$. |
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