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Question
In the following figure, if ∠OAB = 40º, then ∠ACB is equal to ______.

Options
50º
40º
60º
70°
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Solution
In the following figure, if ∠OAB = 40º, then ∠ACB is equal to 50º.
Explanation:

Given: ∠OAB = 40º
Now, in triangle OAB,
OA = OB ...[Radii of circle]
So, ∠OAB = ∠OBA = 40º ...[Angle opposite to equal sides are equal]
So, ∠AOB = 180º – (40º + 40º) = 100º
As we know that angle subtended by an arc of a circle at the center is double the angle subtended by it at any point on the remaining part of the circle.
So, ∠ACB = `1/2` ∠AOB = `1/2 xx 100^circ` = 50º
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