4 CMCD = 5 cmConcept used:In ΔABC, RIGHT angle at BAD is PERPENDICULAR to BC, thenAD2 = BD × DCCalculation:According to propertyAD2 = BD × DC⇒ AD2 = 4 × 5⇒ AD = √20⇒ AD = 2√5 cm∴ The AD is 2√5 cm.