Ineed explaining why triangle pab is isosceles

Its isosceles because 2 sides of the triangle have the same length but  one of the side has a different length so it is classified as an isosceles triangle 

An isosceles triangle is a triangle that has two sides of equal length. Sides PA and PB are equal to each other while AB, the hypotenuse, will only be equal to PA and PB squared. That is the pythogorean therom. Hope this helps.