Applications of Matrices Multiplication to Determinant and Rotations formulas in setR^n

This note deals with two topics of linear algebra. We give a simple and short proof of the multiplicative property of the determinant and provide a constructive formula for rotations. The derivation of the rotation matrix relies on simple matrix calculations and thus can be presented in an elementary linear algebra course. We also classify all invariant subspaces of equiangular rotations in 4D.