Can I trim a cat's fur?

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The people I babysit for have a cat. I'm not sure if he is considered long hair, or medium hair. Anyways, it's long enough go get matted and they do not take him to the groomers. He has a decent size chunk of fur that is matted really bad. Can I just take a pair if scissors and cut it off?

Would brushing his fur regularly help prevent this? If so, how often and what kind of brush?