Question

the focal length of a concave mirror is 30 cm find the position of an object in front of the mirror so that the image is 3 times the size of the object ​

Answer 1

Explanation:

v + 1/u = 1/f

Let the image be real.  v is -ve. u is -ve. f is -ve.

v = 3 u      as magnification m =  -v / -u = v/u

1/(-3u)  + 1/(-u)  = 1/(-f)

-4/3u = 1/(-30)  =>  u = 40 cm  that is 40 cm infront of the mirror.

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Let the image be virtual.    so v/u = -3

v is +ve  u is -ve    f is -ve

v = 3u

1/3u - 1/u = -1/f

-2/3 u  =  -1/30    u = 20 cm  that is 20 cm in front of mirror.

Answer 2

Explanation:

f - -30

m -. -3

so;. -v:u = -3

-v =. -3u

now you can use your mirror formula