Question

how they found that it is right angled at A

Answer 1

Hey!!!

We have concluded

AB² + AC² = BC²

Consider the figure attached

We know about Pythagoras theorem which is only and only possible in a right angle ∆

Thus

By Converse of Pythagoras theorem,

If in any ∆ the sum of the squares of the base and perpendicular is equal to the square of the hypotenuse, then the ∆ is a right angle ∆

Thus angle A = 90°

Hence Verified