Question

The value of $${\cos ^2}{10^ \circ } - \cos {10^ \circ }\cos {50^ \circ } + {\cos ^2}{50^ \circ }$$       is:

A. $$\frac{3}{4} + \cos {20^ \circ }$$
B. $$\frac{3}{4}$$  
C. $$\frac{3}{2}\left( {1 + \cos {{20}^ \circ }} \right)$$
D. $$\frac{3}{2}$$
Answer :   $$\frac{3}{4}$$
Solution :
$$\eqalign{ & {\cos ^2}{10^ \circ } - \cos {10^ \circ }\cos {50^ \circ } + {\cos ^2}{50^ \circ } \cr & = \left( {\frac{{1 + \cos {{20}^ \circ }}}{2}} \right) + \left( {\frac{{1 + \cos {{100}^ \circ }}}{2}} \right) - \frac{1}{2}\left( {2\cos {{10}^ \circ }\cos {{50}^ \circ }} \right) \cr & = 1 + \frac{1}{2}\left( {\cos {{20}^ \circ } + \cos {{100}^ \circ }} \right) - \frac{1}{2}\left[ {\cos {{60}^ \circ } + \cos {{40}^ \circ }} \right] \cr & = \left( {1 - \frac{1}{4}} \right) + \frac{1}{2}\left[ {\cos {{20}^ \circ } + \cos {{100}^ \circ } - \cos {{40}^ \circ }} \right] \cr & = \frac{3}{4} + \frac{1}{2}\left[ {2\cos {{60}^ \circ } \times \cos {{40}^ \circ } - \cos {{40}^ \circ }} \right] \cr & = \frac{3}{4} \cr} $$

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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