here is my problem:

Question

a concave mirror forms a real image at 17.0 cm from the mirror surface along the principal axis. if the corresponding object is at a distance of 36.0 cm, what is the mirror's focal length?

here's my work: 1/f = 1/v + 1/u
where f=focal length=?
u= -36 cm (object distance)
v= -17 cm (image distance)
1/f = -1/17 - 1/36
f= -11/54 cm

is this the correct way to do this? is there any other way that is better? is the answer correct? thanks

a tutor told me the u and v and the computed f should all be positive...
should ijust change the signs..or do i have to solve again ? will the anser remain the same number?

Answer 1

I agree on the positive signs.

17*36/(17+36)=
(17 * 36) / (17 + 36) = 11.5 cm