ABCD is a rhombus such that ∠ACB = 50°. Then, ∠ADB = ?

We know that diagonals of rhombus bisect each other at 90°.

Then, in ΔBOC,

90° + 50° + ∠OBC = 180°        (Angle sum property of triangle)

⇒">⇒⇒ ∠OBC = 180° -">−- 140°

⇒">⇒⇒ ∠OBC = 40°

But ∠OBC = ∠ADB     (Alternate interior angles)

Thus, ∠ADB = 40°

Hence, the corerct option is (a).