$O$ is the centre of the circle. If $\angle ACB=40^{\circ}$, then find $\angle AOB$.
"
Given: In the given figure, $O$ is the centre of the circle. If $\angle ACB=40^{\circ}$.
To do: To find $\angle AOB$.
Solution:
Here, $AB$ is a chord.
Angle subtended by chord at the centre of a circle is double of the angle subtended at it's circumference.
Therefore $\angle AOB=2\angle ACB$
$\Rightarrow \angle AOB=40^{\circ}\times2=80^{\circ}$.
Thus, $\angle AOB=80^{\circ}$.
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