In the figure below, AB = AC = CD. If ∠ADB = 20°, what is the value of ∠BAD?
Explanation:
In ∆ ACD, AC = CD ⇒ ∠CAD = ∠CDA = 20°
In ∆ ABC ∠ACB = ∠CAD + ∠CDA = (20 + 20)° = 40° …(Exterior angle theorem)
Also, AC = AB ⇒ m∠ABC = ∠ACB = 40°
∴ m∠BAD = 180 – 40 – 20 = 120°
Hence, option (d).
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