Question

while deriving mirror or lens formula we use signs of convention.And we also use them while solving numericals.It means we are using sign of convention twice.why does this not affect our answer.How equation works here.

Answer 1

Friend while deriving the mirror formula or lens formula we calculate the relation between object distance image distance and focal length and the formula is calculated based on the positions of object and image using sign convention.
since we are using sign convention here so we put the values of v,u, and f with signs and it does not affect our answer.