[-4, 8, 2]/ABS([-4, 8, 2]) + [1, 2, -4]/ABS([1, 2, -4]) = [- √21/21, 2·√21/7, - √21/7]
k * [- √21/21, 2·√21/7, - √21/7] = [- 1, 6, - 3]
[- 1, 6, - 3] = k * [5a-4, 8-6a, 2-6a] --> a = 2/3 ∧ k = 3/2
Prüfe mal ob die Ebene Ea mit a = 2/3 eine Spiegelebene ist.