Physics

i have a question on thin lens equation
how do i know when di is positive or negative?

also i have an example:
A convex lens has a focal length of 60 cm. A candle is placed 50 cm from the lens. What type of image is formed and how far is the image from the lens?
(Is my answer correct?)

G f=60cm
do=50cm

R di=?

A 1/f=1/do+1/di
1/di=1/f-1/do

S 1/di=1/60-1/50
1/di=-1/300
di=-300

P Therefore, the image is virtual and it is -300 cm away from the lens.

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asked by Sam

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