Question

What happens to the Law of Cosines if the known angle is 90 degrees?

Answer 1

See below.

Explanation:

It transforms to the Pythagoras theorem.

c^2 = a^2 + b^2 - 2ab cos C

If C = 90, cos C = zero so the last term disappears.

c^2 = a^2 + b^2 - 2ab * 0

c^2 = a^2 + b^2.