Circle
The radius of a circle is a line, which starts at the centre of the circle and ends on a point on the circumference. The radius is half the length of the diameter.
Looking at the diagram, since OA and OB are radii of the circle, then OA=OB, therefore this forms an isosceles triangle inside the circle.
Also, note that since triangle AOB is isosceles, then \(\angle OAB=\angle OBA\).
Question
In the circle, calculate the size of \(\angle OBA\).
\(\angle AOC\) is a straight angle, so will add up to \(180^\circ\).
\(\angle AOB=180^\circ-136^\circ=44^\circ\)
Triangle AOB is isosceles, so \(\angle OAB=\angle OBA\) and angles in a triangle will add up to \(180^\circ\).
\(\angle OBA=(180^\circ - 44^\circ)\div2=68^\circ\)
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