AB = B - A = [-2, 3, 1] - [1, 0, -2] = [-3, 3, 3] = 3 * [-1, 1, 1]
a)
α = ARCSIN(([-1, 1, 1] * [3, -2, 2]) / (|[-1, 1, 1]| * |[3, -2, 2]|)) = -24.83989246
b)
N = [1, -2, 2] ⨯ [-7, 5, 1] = [-12, -15, -9] = -3 * [4, 5, 3]
α = ARCSIN(([-1, 1, 1] * [4, 5, 3]) / (|[-1, 1, 1]| * |[4, 5, 3]|)) = 19.06245581
c)
α = ARCSIN(([-1, 1, 1] * [0, 0, 1]) / (|[-1, 1, 1]| * |[0, 0, 1]|)) = 35.26438968