Question

The value of $$\cos \frac{\pi }{{11}} + \cos \frac{{3\pi }}{{11}} + \cos \frac{{5\pi }}{{11}} + \cos \frac{{7\pi }}{{11}} + \cos \frac{{9\pi }}{{11}}$$         is

A. $$0$$
B. $$1$$
C. $$\frac{1}{2}$$  
D. None of these
Answer :   $$\frac{1}{2}$$
Solution :
Use $$\cos \alpha + \cos \left( {\alpha + \beta } \right) + \cos \left( {\alpha + 2\beta } \right) + ..... + \cos \left( {\alpha + \overline {n - 1} \beta } \right)$$
$$\,\,\,\,\, = \frac{{\sin \frac{{n\beta }}{2}}}{{\sin \frac{\beta }{2}}}\cos \frac{{2\alpha + \left( {n - 1} \right)\beta }}{2}.$$
Here, $$\alpha = \frac{\pi }{{11}},\beta = \frac{{2\pi }}{{11}},n = 5.$$

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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