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In the given circle with centre O, angle ABC = 100°, ∠ACD = 40° and CT is a tangent to the circle at C. Find ∠ADC and ∠DCT.

In the circle with centre O, angle ABC = 100°, ∠ACD = 40° and CT is a tangent to the circle at C. Find ∠ADC and ∠DCT. Tangents and Intersecting Chords, Concise Mathematics Solutions ICSE Class 10.

Circles

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Answer

From figure,

⇒ ∠ADC + ∠ABC = 180° [Sum of opposite angles in a cyclic quadrilateral = 180°]

⇒ ∠ADC + 100° = 180°

⇒ ∠ADC = 180° - 100°

⇒ ∠ADC = 80°.

In △ADC,

⇒ ∠ADC + ∠CAD + ∠ACD = 180° [By angle sum property of triangle]

⇒ 80° + ∠CAD + 40° = 180°

⇒ ∠CAD = 180° - 120° = 60°.

From figure,

⇒ ∠DCT = ∠CAD = 60° [Angles in alternate segment are equal].

Hence, ∠DCT = 60° and ∠ADC = 80°.

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