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in Circles by (15.1k points)
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In the given figure, if PA and PB are tangents to the circle with centre O such that ∠APB = 54°, then ∠OAB equals

(a) 36°

(b) 18°

(c) 27°

(d) 36°

1 Answer

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by (49.2k points)
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Best answer

Correct option is (c) 27°

We know that the radius and tangent are perpendicular at their point of contact.

∴  OA ⊥ PA and OB ⊥ PB

∴  ∠OAP = ∠OBP = 90°

In quadrilateral OAPB,

⇒ ∠OAP + ∠APB + ∠OBP + ∠AOB = 360°

⇒ 90° + 54° + 90° + ∠AOB = 360°

⇒ 234° + ∠AOB = 360°

⇒ ∠AOB = 360° − 234°

⇒ ∠AOB = 126°

In △AOB,

⇒ OA = OB              [ Radii of same circle ]

⇒ ∠OAB = ∠OBA      [ Base angles of equal sides are also equal ]

Now,

⇒ ∠OAB + ∠OBA + ∠AOB = 180°

⇒ ∠OAB + ∠OAB + 126° = 180°

⇒ 2∠OAB = 180° − 126°

⇒  2∠OAB = 54°

∴  ∠OAB = 27°

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