Finding unknown when limit is given

Find the unknown length in the given figure.

A plano convex lens of unknown material and unknown focal length is given. With the help of a spherometer we can measure the

A plano-convex lens of unknown material and unknown focal length is given. With the help of a spherometer we can measure the,

A plano convex lens of unknown material and unknown focal length is given. With the help of a spherometer we can measure the

In each of the figures given below, AB||CD . Find the unknown angles, giving reasons