Question

Three expressions are given below :
$$\eqalign{ & {Q_1} = \sin \left( {A + B} \right) + \sin \left( {B + C} \right) + \sin \left( {C + A} \right) \cr & {Q_2} = \cos \left( {A - B} \right) + \cos \left( {B - C} \right) + \cos \left( {C - A} \right) \cr & {Q_3} = \sin A\left( {\cos B + \cos C} \right) + \sin B\left( {\cos C + \cos A} \right) + \sin C\left( {\cos A + \cos B} \right) \cr} $$
Which one of the following is correct ?

A. $${Q_1} = {Q_2}$$
B. $${Q_2} = {Q_3}$$
C. $${Q_1} = {Q_3}$$  
D. All the expressions are different
Answer :   $${Q_1} = {Q_3}$$
Solution :
We take $$Q_3$$ first,
$$\eqalign{ & {Q_3} = \sin A\left( {\cos B + \cos C} \right) + \sin B\left( {\cos C + \cos A} \right) + \sin C\left( {\cos A + \cos B} \right) \cr & = \sin A\cos B + \sin A\cos C + \sin B\cos C + \sin B\cos A + \sin C\cos A + \sin C\cos B \cr & = \sin \left( {A + B} \right) + \sin \left( {B + C} \right) + \sin \left( {C + A} \right) = {Q_1} \cr & \Rightarrow {Q_3} = {Q_1} \cr} $$

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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