Given : $$\angle$$A = 60° and AB = 12 cm
To find : BD = ?
Solution : Since, all the sides of a rhombus are equal, => AB = AD
=> $$\angle$$ABD = $$\angle$$ADB
Now, in $$\triangle$$ABD
=> $$\angle$$ABD + $$\angle$$ADB + $$\angle$$A = 180°
=> 2$$\angle$$ABD = 120°
=> $$\angle$$ABD = $$\angle$$ADB = $$\angle$$A = 60°
=> ABD is equilateral triangle.
=> AB = AD = BD = 12 cm
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