(a) Using the mirror equation and the formula of magnification, deduce that "the virtual image produced by a convex mirror is always diminished in size and is located between the pole and the focus." OR (b) A convex lens of focal length $10\text{ cm}$, a concave lens of focal length $15\text{ cm}$ and a third lens of unknown focal length are placed coaxially in contact. If the focal length of the combination is $+12\text{ cm}$, find the nature and focal length of the third lens, if all lenses are thin. Will the answer change if the lenses were thick ?