Question

F=-15 cm , V=-10 find U=?

Answer 1

Explanation:

1/f=1/v-1/u

1/-15=1/-10-1/u

1/-u=1/-15+1/10

1/-u=(-2+3)/30

1/-u=1/30

1/u=1/-30

u=-30

Therefore u=30cm (infront of the mirror)

Answer 2

If it is a mirror, use mirror formula:

1/v + 1/u = 1/f

If it is a lens, use lens formula:

1/v -1/u = 1/f