θ is acute and 4 sin²θ − 3 = 0. Find cos 2θ.

−1/2. sin²θ = 3/4, so cos 2θ = 1 − 2 sin²θ = 1 − 3/2 = −1/2. (θ = 60°, and cos 120° = −1/2.)

θ is acute and 4 si $n^{2}$ θ − 3 = 0. Find cos 2θ.

Competition practiceTrigonometry

n^{2}

θ = 3/4, so cos 2θ = 1 − 2 si

n^{2}

θ = 1 − 3/2 = −1/2. (θ = 60°, and cos 120° = −1/2.)

Answer: A) −1/2

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