Question

focal length of concave mirror is 15cm what is the distance of image when object is place at 30cm from the mirror.

Answer 1

f = -15cm 
u=-30cm
v=?
1/f=1/u+1/v
1/-15=1/-30 +1/v
1/15-1/30=1/v
-2+1/30=1/v
-1/30=1/v
v=-30cm
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Answer 2

1/u+1/v=1/f
1/-30+1/v=1/-15
1/v=-2+1/30
v=-30cm