Question: The side AC of a $ \Delta ABC $ is extended to D such That $ BC=CD. $ If $ \angle ACB $ is $ 70{}^\circ , $ then $ \angle ADB $ is equal to
Options:
A) $ 35{}^\circ $
B) $ 45{}^\circ $
C) $ 70{}^\circ $
D) $ 110{}^\circ $
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Answer:
Correct Answer: A
Solution:
- $ \angle ACB+\angle BCD=180{}^\circ $ [linear pair]
$ \angle BCD=180{}^\circ -70{}^\circ =110{}^\circ $
In $ \Delta BCD, $ $ BC=CD $
$ \angle CBD=\angle CDB $ … (i)
[angles opposite to equal sides]
Also, $ \angle BCD+\angle CBD+\angle CDB=180{}^\circ $
$ 2\angle CDB=180{}^\circ -\angle BCD $
$ =180{}^\circ -110{}^\circ =70{}^\circ $
$ \therefore $ $ \angle CDB=\angle ADB=\frac{70{}^\circ }{2}=35{}^\circ $