Cosine means complementary sine, the sine of an angle and the cosine of the complementary angle are the same.
sin 75° = cos θ
cos (90° - 75°) = cos θ
cos 15° = cos θ
In general when we have
cos x = cos a
the solution is
x = ±a + 360°k, integer k
Here they just went with x=a, or
θ = 15°
Answer: C θ = 15°
