Skills Handbook - Squaring Numbers and Finding Square Roots - Exercises - Page 889: 11

$\frac{2}{3}$

To find out the square root of a number, we must find the number whose square is the original number. To find the square root of a fraction, we find the square root of the numerator and the denominator. Therefore, $\sqrt {\frac{4}{9}}=\frac{\sqrt 4}{\sqrt 9}=\frac{2}{3}$, because $(\frac{2}{3})^2=\frac{4}{9}$.