Question

a concave mirror of focal length 20 cm is placed at a distance of 45 cm from a wall calculate the distance of an object in order to obtain its real image on the wall

Answer 1

As its a concave mirror hence the mirror eqn is
1/v+1/u=1/f
v=-45cm
f=-29
hence u =fv/v-f
u=(-20×-45)/-45-(-20)
=900/-25= -36 cm is the position of object