Question

An object is placed on the principal axis of a concave mirror of focal length 10cm at a distance of 15cm in front of it

Answer 1

f=10cm and u=15 cm

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Use formula 1/v+1/u=1/f, put the values in here and get the value of v i.e image distance✔✔✔✔✔

Answer 2

According to the question
$f = - 10$
$u = - 15$

Now use the mirror formula,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$