An equilateral triangle ABC is inscribed in a circle with centre O. D is a point on the minor arc BC and ∠CBD = 40º. Find the measure of ∠BCD.

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SSC CGL 2021 Tier-I (Held On : 11 April 2022 Shift 1)
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  1. 40º
  2. 30º
  3. 50º
  4. 20º

Answer (Detailed Solution Below)

Option 4 : 20º
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PYST 1: SSC CGL - English (Held On : 11 April 2022 Shift 1)
25 Qs. 50 Marks 12 Mins

Detailed Solution

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Given:

An equilateral triangle ABC is inscribed in a circle with centre O

∠CBD = 40º

Concept used:

The sum of the opposite angles of a cyclic quadrilateral = 180°

The sum of all three angles of a triangle = 180°

Calculation:

∠ABC = ∠ACB = ∠BAC = 60°  [∵ ΔABC is an equilateral triangle]

Also, ∠BAC + ∠BDC = 180°

⇒ 60° + ∠BDC = 180°

⇒ ∠BDC = 180° - 60° = 120°

Also, ∠CBD + ∠BDC + ∠BCD = 180°

⇒ 40° + 120° + ∠BCD = 180°

⇒ ∠BCD = 180° - 40° - 120° = 20°

∴ The value of ∠BCD is 20°

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