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# If Sin a = 4 5 and Cos B = 5 13 , Where 0 < A, B < π 2 , Find the Value of the Following: Cos (A − B) - CBSE (Science) Class 11 - Mathematics

ConceptTrigonometric Functions of Sum and Difference of Two Angles

#### Question

If $\sin A = \frac{4}{5}$ and $\cos B = \frac{5}{13}$, where 0 < A, $B < \frac{\pi}{2}$, find the value of the following:
cos (A − B)

#### Solution

Given:
$\sin A = \frac{4}{5}\text{ and }\cos B = \frac{5}{13}$
We know that
$\cos A = \sqrt{1 - \sin^2 A}\text{ and }\sin B = \sqrt{1 - \cos^2 B} ,\text{ where }0 < A , B < \frac{\pi}{2}$
$\Rightarrow \cos A = \sqrt{1 - \left( \frac{4}{5} \right)^2} \text{ and }\sin B = \sqrt{1 - \left( \frac{5}{13} \right)^2}$
$\Rightarrow \cos A = \sqrt{1 - \frac{16}{25}}\text{ and }\sin B = \sqrt{1 - \frac{25}{169}}$
$\Rightarrow \cos A = \sqrt{\frac{9}{25}}\text{ and }\sin B = \sqrt{\frac{144}{169}}$
$\Rightarrow \cos A = \frac{3}{5}\text{ and }\sin B = \frac{12}{13}$
Now,
$\cos\left( A - B \right) = \cos A \cos B + \sin A \sin B$
$= \frac{3}{5} \times \frac{5}{13} + \frac{4}{5} \times \frac{12}{13}$
$= \frac{15}{65} + \frac{48}{65}$
$= \frac{63}{65}$

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#### APPEARS IN

RD Sharma Solution for Mathematics Class 11 (2019 to Current)
Chapter 7: Values of Trigonometric function at sum or difference of angles
Ex.7.10 | Q: 1.4 | Page no. 19
Solution If Sin a = 4 5 and Cos B = 5 13 , Where 0 < A, B < π 2 , Find the Value of the Following: Cos (A − B) Concept: Trigonometric Functions of Sum and Difference of Two Angles.
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