Question

If $$\sin \theta = \frac{{12}}{{13}}\left( {0 < \theta < \frac{\pi }{2}} \right){\text{and}}\cos \phi = \frac{3}{5},\left( {\pi < \phi < \frac{{3\pi }}{2}} \right)$$           Then $$\sin \left( {\theta + \phi } \right)$$   will be

A. $$\frac{{ - 56}}{{61}}$$
B. $$\frac{{ - 56}}{{65}}$$  
C. $$\frac{{ 1}}{{65}}$$
D. $$ - 56$$
Answer :   $$\frac{{ - 56}}{{65}}$$
Solution :
$$\eqalign{ & {\text{We have, }}\sin \theta = \frac{{12}}{{13}} \cr & \cos \theta = \sqrt {1 - {{\sin }^2}\theta } = \sqrt {1 - {{\left( {\frac{{12}}{{13}}} \right)}^2}} = \frac{5}{{13}} \cr & {\text{and }}\cos \phi = \frac{{ - 3}}{5}, \sin\phi = \sqrt {1 - \frac{9}{{25}}} = \frac{{ - 4}}{5},\left[ {\because \pi < \phi < \frac{{3\pi }}{2}} \right] \cr & {\text{Now, }}\sin \left( {\theta + \phi } \right) = \sin \theta \cdot \cos \phi + \cos \theta \cdot \sin \phi \cr & = \left( {\frac{{12}}{{13}}} \right)\left( {\frac{{ - 3}}{5}} \right) + \left( {\frac{5}{{13}}} \right)\left( {\frac{{ - 4}}{5}} \right) = \frac{{ - 36}}{{65}} - \frac{{20}}{{65}} = \frac{{ - 56}}{{65}} \cr} $$

Releted MCQ Question on
Trigonometry >> Trigonometric Ratio and Identities

Releted Question 1

If $$\tan \theta = - \frac{4}{3},$$   then $$\sin \theta $$  is

A. $$ - \frac{4}{5}{\text{ but not }}\frac{4}{5}$$
B. $$ - \frac{4}{5}{\text{ or }}\frac{4}{5}$$
C. $$ \frac{4}{5}{\text{ but not }} - \frac{4}{5}$$
D. None of these
Releted Question 2

If $$\alpha + \beta + \gamma = 2\pi ,$$    then

A. $$\tan \frac{\alpha }{2} + \tan \frac{ \beta }{2} + \tan \frac{\gamma }{2} = \tan \frac{\alpha }{2}\tan \frac{\beta }{2}\tan \frac{\gamma }{2}$$
B. $$\tan \frac{\alpha }{2}\tan \frac{\beta }{2} + \tan \frac{\beta }{2}\tan \frac{\gamma }{2} + \tan \frac{\gamma }{2}\tan \frac{\alpha }{2} = 1$$
C. $$\tan \frac{\alpha }{2} + \tan \frac{ \beta }{2} + \tan \frac{\gamma }{2} = - \tan \frac{\alpha }{2}\tan \frac{\beta }{2}\tan \frac{\gamma }{2}$$
D. None of these
Releted Question 3

Given $$A = {\sin ^2}\theta + {\cos ^4}\theta $$    then for all real values of $$\theta $$

A. $$1 \leqslant A \leqslant 2$$
B. $$\frac{3}{4} \leqslant A \leqslant 1$$
C. $$\frac{13}{16} \leqslant A \leqslant 1$$
D. $$\frac{3}{4} \leqslant A \leqslant \frac{{13}}{{16}}$$
Releted Question 4

The value of the expression $$\sqrt 3 \,{\text{cosec}}\,{\text{2}}{{\text{0}}^ \circ } - \sec {20^ \circ }$$     is equal to

A. 2
B. $$\frac{{2\sin {{20}^ \circ }}}{{\sin {{40}^ \circ }}}$$
C. 4
D. $$\frac{{4\sin {{20}^ \circ }}}{{\sin {{40}^ \circ }}}$$

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