Answer:

Explanation:
See the attached diagram.
Given that, Δ ADH is a right triangle with ∠ ADH = 90°.
So, applying Pythagoras Theorem
AH² = AD² + DH² = a² + b²
⇒
{Neglecting the negative root as AH can not be negative}
Therefore, the perimeter of the triangle Δ ADH = AD + AH + DH
⇒
(Answer)