If you are downloading the official, legitimate version of Fast Screen Recorder 2.0.0.5 from the developer's website or a trusted archive, it is generally safe. However, the moment you search for "Pre-Activated" versions on third-party "warez" sites, torrent trackers, or dubious forums, you are gambling with your cybersecurity.
You need to record 20 short math tutorials. Fast Screen Recorder lets you hit F9 to start, F9 to stop. No configuration needed between takes. The pre-activated nature means no "Trial Expired" panic mid-semester.
OBS Studio is the industry standard for free screen recording and streaming. It is open-source, completely free, and has no watermarks or recording time limits. While it has a steeper learning curve than Fast Screen Recorder, it is far more powerful and secure.