Question

In Mirror formula, u = object distance and f = focal length. Then u = 2f ? Please give reason.

Answer 1

It's not necessary that u should be 2f only, u can be anything from zero to infinity.
u is 2f only when the object is placed at centre of curvature.

Answer 2

not necessarily
if u = 2f , then the object is at C and the image is also formed at C where image is real,inverted and same size of the object.