Question

The value of $${\sin ^2}{5^ \circ } + {\sin ^2}{10^ \circ } + {\sin ^2}{15^ \circ } + \sin {20^ \circ } + ..... + {\sin ^2}{90^ \circ }{\text{ is}}$$

A. $$7$$
B. $$8$$
C. $$9$$
D. $$\frac{{19}}{2}$$  
Answer :   $$\frac{{19}}{2}$$
Solution :
$$\eqalign{ & {\sin ^2}{5^ \circ } + {\sin ^2}{10^ \circ } + {\sin ^2}{15^ \circ } + ..... + ..... + {\sin ^2}{75^ \circ } + {\sin ^2}{80^ \circ } + {\sin ^2}{85^ \circ } + {\sin ^2}{90^ \circ } \cr & \Rightarrow {\sin ^2}{5^ \circ } + {\sin ^2}{10^ \circ } + {\sin ^2}{15^ \circ } + ..... +{\sin ^2}\left( {{{90}^ \circ} - {{15}^ \circ }} \right) + {\sin ^2}\left( {{{90}^ \circ} - {{10}^ \circ}} \right) + {\sin ^2}\left( {{{90}^ \circ} - {{5}^ \circ}} \right) + 1 \cr & \Rightarrow {\sin ^2}{{5}^ \circ} + {\sin ^2}{{10}^ \circ} + {\sin ^2}{15^ \circ } + ..... + {\cos ^2}{15^ \circ } + {\cos ^2}{10^ \circ } + {\cos ^2}{5^ \circ } + 1 \cr & \Rightarrow \left( {1 + 1 + 1 + .....8\,{\text{times}}} \right) + \sin {45^ \circ } + 1 \cr & \Rightarrow 8 + \frac{1}{2} + 1 = \frac{{19}}{2} \cr} $$

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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