Step-by-step explanation:
cos(90) = 0
around this point the cos function "mirrors" with opposite signs. cos(<90) is positive and cos(>90) is negative.
but |cos(90-a)| = |cos(90+a)| for 0 <= a <= 90
75 = 90 - 15
105 = 90 + 15
so, a = 15
and because of
|cos(90-a)| = |cos(90+a)| for 0 <= a <= 90
cos (90-15) = cos(75) = -cos(90+15) = -cos(105)
