@mathladyhazel That is a pendulum and circular movement, both are related to the sin(x) function and that is the reason why the balls don't hit each other.
@pickover An isosceles triangle has two equal sides, therefore, their opposite angles are equal as well. Hence:
78° + 78° + (x!)° = 180°, then:
(x!)° = 24°
If is x= 4, we have:
4! = 4x3x2x1 = 24, therefore:
x =4.