My top favorite of my answers (sorted with easiest to understand first):
rank 2: Matrix Representation
3-D Imaging: simplifying equations
Sub-Matrix: Efficient Inverse
QR: Fundamental Subspaces
Sylvester Equation: Explicit $2 \times 2$
Pseudo-inverse: Efficient and simple calculation
Unitary Similarity: python code to "hollow a matrix"
Furthest in angle vector: "Box Rotation"
norm as symmetric and skew symmetric