Question

If the term [uv / (u+v)] = - 18 cm, then R = ____ cm

(a) -18 (b) -36 (c) -9 (d) Insufficient data

Answer 1

Answer:

[uv / (u+v)] = - 18 cm

or, 1÷ [uv / (u+v)] = 1÷ (-18)cm

or, (u+v)/uv = -(1/18)cm

or, (u/uv)+(v/uv) = -(1/18)cm

or, (1/v)+ ( 1/u) = -(1/18)cm

Mirror formula----

(1/v)+(1/u)= (1/f)

therefore, -18cm must be mirror's focal length

R=2f

therefore, R= 2(-18) cm = -36 cm

b is the correct answer