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The function f(x)=log⁡e(sin⁡x),x∈(0,π)f(x) = \log_e(\sin x), x \in (0, \pi) is

(A) strictly increasing on (0,π2)\left(0, \frac{\pi}{2}\right)

(B) strictly decreasing on (0,π2)\left(0, \frac{\pi}{2}\right)

(C) strictly increasing on (π2,π)\left(\frac{\pi}{2}, \pi\right)

(D) strictly decreasing on (π2,π)\left(\frac{\pi}{2}, \pi\right)

(E) strictly increasing on (0,π)(0, \pi)

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 1

To determine where f(x)=log⁡e(sin⁡x)f(x) = \log_e(\sin x) is increasing or decreasing, find the derivative.

Using the chain rule:

f(x)=log⁡e(sin⁡x)f(x) = \log_e(\sin x)

f′(x)=1sin⁡x×cos⁡xf'(x) = \frac{1}{\sin x} \times \cos x

f′(x)=cos⁡xsin⁡xf'(x) = \frac{\cos x}{\sin x}

f′(x)=cot⁡xf'(x) = \cot x


For interval (0,π2)(0, \frac{\pi}{2}):

In this interval, sin⁡x>0\sin x > 0 and cos⁡x>0\cos x > 0.

Therefore cot⁡x=cos⁡xsin⁡x>0\cot x = \frac{\cos x}{\sin x} > 0

Since f′(x)>0f'(x) > 0, the function is strictly increasing on (0,π2)(0, \frac{\pi}{2}).

Statement (A) is correct.


For interval (π2,π)(\frac{\pi}{2}, \pi):

In this interval, sin⁡x>0\sin x > 0 and cos⁡x<0\cos x < 0.

Therefore cot⁡x=cos⁡xsin⁡x<0\cot x = \frac{\cos x}{\sin x} < 0

Since f′(x)<0f'(x) < 0, the function is strictly decreasing on (π2,π)(\frac{\pi}{2}, \pi).

Statement (D) is correct.


Statements (B), (C), and (E) are incorrect based on the analysis above.

The correct answer is (A) and (D).

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