Answer:
cos(θ) = -3/5
Explanation:
The cosine of the reference angle (in the first quadrant) is ...
cos(θ) = √(1 -sin(θ)²) = √(1 -(4/5)²) = √(1 -16/25) = √((25-16)/25)
= √(9/25) = 3/5 . . . . in the first quadrant
In the second quadrant, the cosine is negative, so the answer is ...
cos(θ) = -3/5