Mathematics
https://keisan.casio.com/menu/system/000000000250

Calcuate distance from foot of perpendicular to line
https://math.stackexchange.com/questions/33868/foot-of-perpendicular-to-line

How do you find the distance from a point to a plane?
https://math.stackexchange.com/questions/88392/how-do-you-find-the-distance-from-a-point-to-a-plane

How to find the equation of a plane using three non-collinear points
https://www.maplesoft.com/support/help/maple/view.aspx?path=MathApps%2FEquationofaPlane3Points

Find the equation of the plane knowing that it passes through 3 points
https://math.stackexchange.com/questions/1156983/find-the-equation-of-the-plane-knowing-that-it-passes-through-3-points

Find plane by normal and instance point + distance between origin and plane
https://math.stackexchange.com/questions/752252/find-plane-by-normal-and-instance-point-distance-between-origin-and-plane

Linear Algebra

Fourier

Fourier Transform
https://www.youtube.com/watch?v=spUNpyF58BY

Mandelbrot

Times Tables, Mandelbrot and the Heart of Mathematics
https://www.youtube.com/watch?v=qhbuKbxJsk8

Tutorials Sites

Sites

Immersive Linear Algebra
http://immersivemath.com/ila/learnmore.html

YouTube Channels

3Blue1Brown
https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw

StatQuest with Josh Starmer
https://www.youtube.com/channel/UCtYLUTtgS3k1Fg4y5tAhLbw

Mathologer
https://www.youtube.com/channel/UC1_uAIS3r8Vu6JjXWvastJg