For optics in our NEET preparation, we follow the New Cartesian Sign Convention. The minus sign for the focal length of a concave mirror or a concave lens comes because they diverge light, with the focus being on the same side as the object. It is simply a rule to make calculations consistent.
I remember this clearly from the sticky notes on my wall, next to my biology diagrams, because my bhai, who is an engineer, tested me relentlessly on it over cups of chai, saying even a future doctor must not confuse her signs.
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