Θ lies in quadrant III, φ lies in quadrant I. cos θ = -8 /17 , cos φ = 15 /17 . What is cos (θ - φ)?

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

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step-by-step explanation:

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