Answer:
cos(θ - φ) is -240/289
Step-by-step explanation:
The given parameters are;
The location of θ = Quadrant III
The location of φ = Quadrant I
cos(θ) = -8/17
cos(φ) = 15/17
By trigonometric identities, we have;
cos(θ - φ) = cos(θ)·cos(φ) - sin(θ)·sin(φ)
Given that 'θ' is in quadrant III, we have;
sin(θ) is negative
By Pythagoras' theorem, we have;
∴ sin(θ) = -√(17²- 8²)/17 = -15/17
Whereby 'φ' is in quadrant I, by Pythagoras' theorem, we have;
sin(φ) = √(17² - 15²)/17 = 8/17
∴ cos(θ - φ) = (-8/17) × (15/17) + (-15/17) × (8/17) = -240/289 = -0.83044982699
