The videos are a supplement to the lectures, and not a replacement. To find the videos go the the course schedule. There you will find all the videos. Remember that you need to log in in order to see the videos.
Alternatively you could also go to this page:
/studier/emner/matnat/its/TEK4030/h21/forelesningsvideoer/
As the course do not have a digital blackboard this year, it might be easier to follow mathematical derivation using last year's videos. They are available here: