In lenses chapter,(i.e.lens law)
When do you take object distance as negative
Object distance (u) is always kept on the left side of the mirror so acc. tp sign convention it is always negative. Therefore for concave n convex mirror, the object distance is always negative.
Image distance (v) is always positive for convex mirror as the image formed is virtual and erect and on the right side of the mirror. In concave mirror only in the case when object is placed b/w F and P, the image formed will be virtual n erect and image distance will be positive. In rest all cases image distance is real n inverted so image distance will be negative.
Therefore, we conclude that If image is real and inverted then image distance is negative and if the image is virtual and erect then the image distance will be positive.