Question

Given: sinθ = -3/5, θ is a third quadrant angle, and tan φ = -7/24, φ is a second-quadrant angle; find cos(θ + φ])

-117/125
-75/125
117/125

Answer 1

First option: cos(θ + φ) = -117/125

Explanation:

Recall that cos(θ + φ) = cos(θ)cos(φ) - sin(θ)sin(φ)

If sin(θ) = -3/5 in Quadrant III, then cos(θ) = -4/5.

Since tan(φ) = sin(φ)/cos(φ), then sin(φ) = -7/25 and cos(φ) = 24/25 in Quadrant II.

Therefore:

cos(θ + φ) = cos(θ)cos(φ) - sin(θ)sin(φ)

cos(θ + φ) = (-4/5)(24/25) - (-3/5)(-7/25)

cos(θ + φ) = (-96/125) - (21/125)

cos(θ + φ) = -96/125 - 21/125

cos(θ + φ) = -117/125