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If Tan θ = Sin α − Cos α Sin α + Cos α , Then Show that Sin α + Cos α = √ 2 Cos θ . - CBSE (Science) Class 11 - Mathematics

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Question

If \[\tan\theta = \frac{\sin\alpha - \cos\alpha}{\sin\alpha + \cos\alpha}\] , then show that \[\sin\alpha + \cos\alpha = \sqrt{2}\cos\theta\].

Solution

\[\tan\theta = \frac{\sin\alpha - \cos\alpha}{\sin\alpha + \cos\alpha}\]
Dividing numerator and denominator on the RHS by \[\cos\alpha\], we get 

\[\tan\theta = \frac{\frac{\sin\alpha}{\cos\alpha} - 1}{\frac{\sin\alpha}{\cos\alpha} + 1}\]

\[ \Rightarrow \tan\theta = \frac{\tan\alpha - \tan\frac{\pi}{4}}{1 + \tan\alpha \tan\frac{\pi}{4}}\]

\[ \Rightarrow \tan\theta = \tan\left( \alpha - \frac{\pi}{4} \right)\]

\[ \Rightarrow \theta = \alpha - \frac{\pi}{4}\]

\[\text{ Or }\alpha = \frac{\pi}{4} + \theta\]
Now,
\[\sin\alpha + \cos\alpha\]
\[ = \sin\left( \frac{\pi}{4} + \theta \right) + \cos\left( \frac{\pi}{4} + \theta \right)\]
\[ = \sin\frac{\pi}{4}\cos\theta + \cos\frac{\pi}{4}\sin\theta + \cos\frac{\pi}{4}\cos\theta - \sin\frac{\pi}{4}\sin\theta\]
\[ = \frac{1}{\sqrt{2}}\cos\theta + \frac{1}{\sqrt{2}}\sin\theta + \frac{1}{\sqrt{2}}\cos\theta - \frac{1}{\sqrt{2}}\sin\theta\]
\[ = \frac{2}{\sqrt{2}}\cos\theta\]
\[ = \sqrt{2}\cos\theta\]
\[\therefore \sin\alpha + \cos\alpha = \sqrt{2}\cos\theta\]

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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: 33 | Page no. 21
Solution If Tan θ = Sin α − Cos α Sin α + Cos α , Then Show that Sin α + Cos α = √ 2 Cos θ . Concept: Trigonometric Functions of Sum and Difference of Two Angles.
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