Question

simplify the radical expression by rationalizing the denominator
4/√21

Answer 1

I think

$\dfrac{4}{\sqrt{21}}$

is about as simple as this is going to get. For some reason teachers like the radicals in the numerator, so we multiply top and bottom by $\sqrt{21}$ and get

$\dfrac{4}{\sqrt{21}} \times \dfrac{\sqrt{21}}{\sqrt{21}} = \dfrac{4\sqrt{21}}{21}$

Answer: $\dfrac{4\sqrt{21}}{21}$

Answer 2

$\frac{4}{\sqrt{21}} =\frac{4\sqrt{21}}{\sqrt{21}\sqrt{21}} =\frac{4\sqrt{21}}{21}$