For each of the following values of sinθ, determine the measure of θ if,
/2 ≤ θ ≤ 3/2
a) 1/2
for $\displaystyle \sin(\theta) = \frac{1}{2}$
$\displaystyle \theta = \frac{\pi}{6}$
Look here Special Triangles
but this answer is not in the domain you require.
Use the identity $\displaystyle \pi -\theta$ to get your answer.