AB is a chord of a circle with centre O. C is a point on the circumference of the circle in the minor sector. If ∠ABO = 40°, what is the measure (in degree) of ∠ACB?

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SSC CGL 2021 Tier-I (Held On : 19 April 2022 Shift 3)
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  1. 110°
  2. 120°
  3. 130°
  4. 100°

Answer (Detailed Solution Below)

Option 3 : 130°
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Detailed Solution

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Given:

∠ABO = 40°

Concept:

The angle subtended by an arc at the center is double than subtended by it on other parts of the circle. In the figure below, ∠ AOB = 2∠ACB

 

Calculation:

In the figure ∠ABO = ∠BAO (angles formed by the radius are equal)

In Δ AOB, ∠ABO + ∠BAO + ∠BOA = 180°

⇒ ∠BOA = 180° - (40 + 40) = 100°

⇒ ∠BOA (external) = 360° - 100° = 260°.

∠ACB =  ∠BOA (external) (since angle formed by the arc at the center is double compared to the angle formed by the arc at other points on the circumference).

∴ ∠ACB =  × 260° = 130°

The angle formed by ∠ACB is 130°.

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