Question

 A 3 cm tall object is placed 18 cm in front of a concave mirror of focal length 12 cm. At what distance from the mirror should a screen be placed to see a sharp image of the object on the screen. Also calculate the height of the image formed. 

Answer 1

Here
u= -18cm
f= -12cm
$1 \div f = (1 \div v ) - (1 \div u)$
So (1/-12)+(1/-18)=1/v
=-10/72
so v= -7.2cm