Sign is always taken -(negative) from pole of spherical mirror to the object along the principle axis.This means sign is - in front of the mirror. And likewise its positive behind the spherical mirror.
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   It is the sign convention adopted for expressing one relationship among the physical quantities f, u and v.  The convention allows an easy and one formula for both convex and concave mirrors.

   Quantities are positive if measured in the direction of the light rays emanating from the object (real or virtual).

   Quantities are negative if measured in the opposite direction.

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but why
to have the same formula for both mirrors. a sign convention is defined. u can define what ever you want.