A virtual image three times the size of the object is obtained with a concave mirror of radius of curvature 36 cm. The distance of the object from the mirror is 

Given:
Magnification (m) = 3 (virtual image is erect).

Radius of the curvature, R =36 cm
Focal Length (f) =
= -18cm (Negative for concave mirror).

Now by formula,

1. Magnification, 

Where,

v is the image distance,

u is the object distance,

Putting the values in the above formula, we get

 

2. Mirror formula

Where
f is the focal length of the mirror
v is the image distance from the mirror
u is the object distance from the mirror
Thus, putting the values in the equation, we get

Thus, on solving the above equation, we get

u=-12 cm

Hence the distance of the object from the mirror is 12 cm. 

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