Definition: An orthogonal matrix is a square matrix whose columns are orthonormal. Determine if each of the following statements is TRUE or FALSE. If true, please briefly explain why. If false, please provide a counterexample.
(a) If Q is an orthogonal matrix, then Q-1 is orthogonal.
(b) If Q is an orthogonal matrix, then QT is orthogonal.
(c) If Q1 and Q2 are orthogonal matrices, then Q1Q2 is orthogonal.
(d) If Q is a matrix with orthonormal columns (need not be square), then ||Qx|| = ||x|| for every x.
(e) If Q is a matrix with orthonormal columns (need not be square), then QQT is the identity matrix.