Question

If an angle $$B$$ is complement of an angle $$A,$$ what are the greatest and least values of $$\cos A \cos B$$   respectively ?

A. $$0, - \frac{1}{2}$$
B. $$\frac{1}{2}, - 1$$
C. $$1, 0$$
D. $$\frac{1}{2}, - \frac{1}{2}$$  
Answer :   $$\frac{1}{2}, - \frac{1}{2}$$
Solution :
Since, $$A$$ and $$B$$ are complementary angles, then $$A + B = {90^ \circ }$$
Now, $$\cos A\cos B = \cos A\cos \left( {{{90}^ \circ } - A} \right)$$
$$ = \cos A\sin A = \frac{1}{2}\sin 2A$$
Since, $$ - 1 \leqslant \sin 2A \leqslant 1$$
Hence, $$ - \frac{1}{2} \leqslant \frac{1}{2}\sin 2A \leqslant \frac{1}{2}.$$
Thus, greatest and least values of $$\cos A \cos B$$   are $$ \frac{1}{2}$$ and $$ - \frac{1}{2} .$$

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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