Question

If $$\tan \frac{\alpha }{2}$$  and $$\tan \frac{\beta }{2}$$  are the roots of the equation $$8{x^2} - 26x + 15 = 0\,$$    then $$\cos \left( {\alpha + \beta } \right)$$   is equal to

A. $$ - \frac{{627}}{{725}}$$  
B. $$ \frac{{627}}{{725}}$$
C. $$- 1$$
D. None of these
Answer :   $$ - \frac{{627}}{{725}}$$
Solution :
Use $$\cos \left( {\alpha + \beta } \right) = \frac{{1 - {{\tan }^2}\frac{{\alpha + \beta }}{2}}}{{1 + {{\tan }^2}\frac{{\alpha + \beta }}{2}}},\tan \frac{{\alpha + \beta }}{2} = \frac{{\tan \frac{\alpha }{2} + \tan \frac{\beta }{2}}}{{1 - \tan \frac{\alpha }{2} \cdot \tan \frac{\beta }{2}}}$$
and $$\tan \frac{\alpha }{2} + \tan \frac{\beta }{2} = \frac{{13}}{4},\tan \frac{\alpha }{2}\tan \frac{\beta }{2} = \frac{{15}}{8}.$$

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. $$\frac{{4\sin {{20}^ \circ }}}{{\sin {{40}^ \circ }}}$$

Practice More Releted MCQ Question on
Trigonometric Ratio and Identities


Practice More MCQ Question on Maths Section