9. why it is advised to breathe through the nose only and not through the mouth?
10. what should be the position of our face if we want to use concave mirrors as shaving mirrors. give reason

Because the nostrils are smaller than the mouth, air exhaled through the nose creates a back flow of air (and oxygen) into the lungs. And because we exhale more slowly through the nose than we do though the mouth, the lungs have more time to extract oxygen from the air we've already taken in.

10. You would have to put your face in front of the mirror because you would have to be positioned at a ratio of 1:1 sense it’s caved