Answer
$\dfrac{3\sqrt{10}}{10}$
Work Step by Step
From problem 21 $\to \cos{A} = -\dfrac{4}{5}$
Using the half angle formula
$\sin{\dfrac{A}{2}} = \sqrt{\dfrac{1-\cos{A}}{2}} = -\sqrt{\dfrac{1+\dfrac{4}{5}}{2}}$
$\sin{\dfrac{A}{2}} = \dfrac{3\sqrt{10}}{10}$