Step 1: Find Angle OSQ.
In the circle, OQ and OS are radii. Therefore, OQ=OS.
This means that △OQS is an isosceles triangle.
In an isosceles triangle, the angles opposite the equal sides are equal: ∠OQS=∠OSQ.
The sum of angles in a triangle is 180∘.
We are given ∠QOS=108∘.
∠QOS+∠OQS+∠OSQ=180∘
108∘+∠OSQ+∠OSQ=180∘