Answer
$\sin (\dfrac{\theta}{2}) =\sqrt {\dfrac{1-\cos \theta}{2}}$
Work Step by Step
We know that the half-angle formula for $\sin$ is $\sin (\dfrac{x}{2}) = \pm \sqrt {\dfrac{1-\cos x}{2}}$
Since the angle is acute, we will write it as:
$\sin (\dfrac{\theta}{2}) =\sqrt {\dfrac{1-\cos \theta}{2}}$