Question

An object is placed on the principal axis of a concave lens at a distance of 20 centimeter from it. If the focal length of the lens is also 20 centimeter , find the location of the image.

Answer 1

u = -20 cm
 f = -20 cm
 v= ?
applying the lens formula, i.e, 
1/v - 1/u = 1/f
putting the values of u and f 
1/v -(-1/20) = -1/20
1/v + 1/20 = -1/20
1/v = -1/10
v = -10 cm

therefore, the image is formed at a distance 10 cm from the optical  center and is a virtual erect and diminished image.

Answer 2

Answer:

given:

u= -20

f= -20

applying formula

1/v - 1/u = 1/f

1/v - (1/-20) = 1/-20

1/v + 1/20 = 1/-20

1/v = 1/-20 - 1/20

1/v = -2/20

1/v = -1/10

v= -10 cm

Hope its help