Single Choice

Left $$f$$ be an odd function defined on the set of real numbers such that for $$x\geq 0, f(x)=3\sin x+4\cos x$$. Then $$f(x)$$ at $$x= \displaystyle -\frac{11\pi}{6}$$ is equal to :

A$$\displaystyle -\frac{3}{2} - 2 \sqrt{3}$$
B$$\displaystyle \frac{3}{2}-2\sqrt{3}$$
Correct Answer
C$$\displaystyle \frac{3}{2}+2\sqrt{3}$$
D$$\displaystyle -\frac{3}{2}+2\sqrt{3}$$

Solution

$$f(x) = 3 \sin x + 4 \cos x $$ $$x \geq 0$$
$$f(x) =5 \sin (x+\alpha)$$
$$\alpha = \sin^{-1}\left ( \dfrac{4}{5} \right )$$
$$f\left

( \dfrac{-11 \pi}{6}\right ) = -f \left ( \dfrac{11\pi}{6} \right ) =-[3

\sin \left ( \dfrac{11 \pi}{6}\right ) + 4 \cos \left ( \dfrac{11 \pi}{6}

\right )]$$
$$=-\left [ 3 \sin (2 \pi - \dfrac{\pi}{6})+4 \cos (2\pi - \dfrac{\pi}{6})\right ]$$
$$=-\left [ -\dfrac{3}{2}+2\sqrt{3} \right ]=\dfrac{3}{2} - 2 \sqrt{3}$$


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