We are given that,

∆ABC is a right-angled triangle at C.

Let the sides of the ∆ABC be

AC = b

BC = a

AB = c

By Pythagoras theorem, where C is the right angle,

(AC)² + (BC)² = (AB)²

⇒ b² + a² = c²

Or,

a² + b² = c² …(i)

Using the property of inverse trigonometry,

Replacing A by and B by ,

Substituting the value of a² + b² from equation (i),

=tan^-1