# The mysterious angle

We have the following configuration where the triangle **OAB **is isosceles and the angles **lambda = 20 °**, **alpha = 60 °** and **beta = 50 °** are given.

It is a question of finding the value of gamma.

## Make a correct drawing

The drawing provided is grossly wrong. By applying yourself with a ruler and a protractor, you will find **80 °**, unless that is a very close number of course. How to know ? There is only one solution for that: you have to prove it!