In rhombus ABCD
∠ACB = 40° given
∠ACB = ∠CAD = 40° [Alternate interior angles are equal]
In ΔAOD
∠AOD = 90° [In rhombus diagonals bisect eact other at right angles]
∠AOD + ∠CAD +∠ADB = 180° [Angle sum property of a triangle]
90° + 40° + ∠ADB = 180°
∠ADB = 180°-130°
∠ADB = 50°
