Question

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Answer 1

f = - 15cm
v = - 10cm
1/v - 1/u = 1/f
-1/u = -1/15+1/10
= -2+3/30
= -30 cm

Answer 2

It's easy
f=-15
V=-10
U=?
1/f =1/v -1/u
1/u=1/v -1/f
1/u = 1/-10 - 1/-15
= -15+10/150= -5/150 = - 1/30
Therefore u=-30