Determine the focal length of a concave mirror if:
(a) with the distance between an object and its image being equal
to 1 = 15 cm, the transverse magnification 6 = —2.0;
(b) in a certain position of the object the transverse magnification
is Ni = —0.50 and in another position displaced with respect to the
former by a distance 1 = 5.0 cm the transverse magnification 62 =
= —0.25.