differential equation

1. If vectors in a real inner product space have equal length, then are orthogonal.
Prove it, using the general properties of the inner product.
2. Show that eigenvalues of any real skew-symmetric (matrix are either 0 or pure imaginary.
(Recall the proof of reality for the eigenvalues of a symmetric matrix, modify it slightly)
3. Find the general solution of the system, of course in real form