CBSE Class 9 Answered

Suppose for the sake of argument that the minimum deviation occurs when the entrance and exit angles are not equal. In a ray diagram, you can always reverse the direction of a light ray and get another valid light ray. In this case, reversing the ray switches the values of the entrance and exit angles. So there are two different values for the entrance angle that give minimum deviation. But if there's only one minimum, this can't be true. Therefore the initial supposition must be false, and the entrance and exit angles must be equal at minimum deviation. The other part follows from the symmetry arguement.