Question:medium

If \(\cos A = \frac{1}{7}\) and \(\cos B = \frac{13}{14}\), then \(\cos (A - B)\) is

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When using trigonometric identities, ensure you calculate \(\sin A\) and \(\sin B\) correctly when only the cosines are given.
Updated On: Jan 15, 2026
  • 1
  • 13/98
  • 1/2
  • 18/49
Show Solution

The Correct Option is D

Solution and Explanation

Apply the cosine difference formula: \[\n\cos (A - B) = \cos A \cos B + \sin A \sin B\n\] Using \(\cos A = \frac{1}{7}\) and \(\cos B = \frac{13}{14}\), compute \(\sin A = \sqrt{1 - \cos^2 A}\) and \(\sin B = \sqrt{1 - \cos^2 B}\). The solution yields: \[\n\cos (A - B) = \frac{18}{49}\n\] The answer is \(18/49\).
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