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Multiple Choice 1 point Considering only the values of \( \alpha \) for which \( \tan \alpha \cos (2 \alpha) \) is defined, which of the following expressions is equivalent to tan \( \alpha \cos (2 \alpha) \) ? \( 2 \cos ^{2} \alpha \) \( \cos \alpha \cot \alpha-\sin \alpha \) \( \frac{2 \sec \alpha}{1-\tan ^{2} \alpha} \) \( 2 \sin \alpha \cos \alpha-\tan \alpha \) \( 2 \sin ^{2} \alpha-\tan ^{2} \alpha \)

Ask by Flynn Gardner. in the United States
Feb 26,2025

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Tutor-Verified Answer

Answer

The correct expression equivalent to tan α cos 2α is \(2 \sin \alpha \cos \alpha - \tan \alpha\).

Solution

We start with the expression tan α cos 2α and recall that tan α = sin α/cos α. Thus we have   tan α cos 2α = (sin α/cos α) cos 2α = (sin α cos 2α)/cos α. Now, consider option 4: 2 sin α cos α - tan α. We rewrite tan α in this expression as sin α/cos α:   2 sin α cos α - tan α = 2 sin α cos α - (sin α/cos α). To combine these terms with a common denominator, express 2 sin α cos α as (2 sin α cos² α)/cos α:   = (2 sin α cos² α - sin α)/cos α   = sin α (2 cos² α - 1)/cos α. Recall that the double-angle identity for cosine is cos 2α = 2 cos² α - 1. Therefore,   sin α (2 cos² α - 1)/cos α = (sin α cos 2α)/cos α. This is exactly the expression we derived for tan α cos 2α. Hence, option 4 is equivalent to tan α cos 2α. Thus, the correct answer is:   2 sin α cos α - tan α.

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To determine which expression is equivalent to \( \tan \alpha \cos (2 \alpha) \), we can first rewrite \( \tan \alpha \) and \( \cos(2\alpha) \): \[ \tan \alpha = \frac{\sin \alpha}{\cos \alpha}, \quad \cos(2\alpha) = \cos^2 \alpha - \sin^2 \alpha = 2\cos^2 \alpha - 1 \] Thus, \[ \tan \alpha \cos(2 \alpha) = \frac{\sin \alpha}{\cos \alpha} (2 \cos^2 \alpha - 1) = \sin \alpha \left( \frac{2 \cos^2 \alpha - 1}{\cos \alpha} \right) = \sin \alpha (2 \cos \alpha - \tan \alpha) \] Now let's look at the options: 1. \( 2 \cos^2 \alpha \) - does not represent \( \tan \alpha \cos(2\alpha) \). 2. \( \cos \alpha \cot \alpha - \sin \alpha = \frac{\cos \alpha}{\tan \alpha} - \sin \alpha = 0 \) is not equal to \( \tan \alpha \cos(2\alpha) \). 3. \( \frac{2 \sec \alpha}{1-\tan^2 \alpha} = \frac{2/\cos \alpha}{1 - \sin^2/\cos^2} = 2\cos^2\alpha \), not equivalent. 4. \( 2 \sin \alpha \cos \alpha - \tan \alpha = \sin(2\alpha) - \tan \alpha \) 5. \( 2 \sin^2 \alpha - \tan^2 \alpha = 2 \sin^2 \alpha - \frac{\sin^2 \alpha}{\cos^2 \alpha} \) The correct answer is \( \cos \alpha \cot \alpha - \sin \alpha \).

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