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Question

In Fig. 104, AB || CO, ∠OAB = 150° and ∠OCO = 120°. Then, ∠AOC =
(a) 80°
(b) 90°
(c) 70°
(d) 100°

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Solution

Construction: Draw a line OE from the point O parallel to AB and CD


Since, AB || OE
∴ ∠BAO + ∠AOE = 180 [Angles on the same side of a transversal line are supplementary]
⇒ 150 + ∠AOE = 180
⇒ ∠AOE = 30
Again, CD || OE
∴ ∠DCO + ∠COE = 180 [Angles on the same side of a transversal line are supplementary]
⇒ 120 + ∠COE = 180
⇒ ∠COE = 60
Now, ∠AOC = ∠AOE + ∠COE
= 30 + 60
= 90
Hence, the correct answer is option (b).

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