what is the lens/mirror formula?

Question

some places it is given as "1/f = 1/v - 1/u" , other places it is given as "1/f = 1/v + 1/u". which is true. Also, when to use which, and why is the negative sign there? in each formula, sign convention for the values substituted for each should be negatiev or positive ( with relation to image direction from lens or mirror, etc.,)

Answer 1

the negative sign is there to tell you whether the image is inverted or right-size up.