The problem asks us to verify the following trigonometric identity: $\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{1}{2(1 + \sin x + \cos x)}$.

AlgebraTrigonometryTrigonometric IdentitiesAlgebraic Manipulation
2025/6/15

1. Problem Description

The problem asks us to verify the following trigonometric identity:
cosx1+sinx+sinx1+cosx=12(1+sinx+cosx)\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{1}{2(1 + \sin x + \cos x)}.

2. Solution Steps

First, let's find a common denominator for the left-hand side (LHS) of the equation:
cosx1+sinx+sinx1+cosx=cosx(1+cosx)+sinx(1+sinx)(1+sinx)(1+cosx)\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{\cos x (1 + \cos x) + \sin x (1 + \sin x)}{(1 + \sin x)(1 + \cos x)}
Expanding the numerator, we get:
cosx+cos2x+sinx+sin2x\cos x + \cos^2 x + \sin x + \sin^2 x
We know that sin2x+cos2x=1\sin^2 x + \cos^2 x = 1. So, the numerator becomes:
cosx+sinx+1\cos x + \sin x + 1
Expanding the denominator, we get:
(1+sinx)(1+cosx)=1+cosx+sinx+sinxcosx(1 + \sin x)(1 + \cos x) = 1 + \cos x + \sin x + \sin x \cos x
Therefore, the LHS becomes:
1+sinx+cosx1+sinx+cosx+sinxcosx\frac{1 + \sin x + \cos x}{1 + \sin x + \cos x + \sin x \cos x}
Now, let's work with the right-hand side (RHS) of the equation:
12(1+sinx+cosx)\frac{1}{2(1 + \sin x + \cos x)}
We want to show that
1+sinx+cosx1+sinx+cosx+sinxcosx=12(1+sinx+cosx)\frac{1 + \sin x + \cos x}{1 + \sin x + \cos x + \sin x \cos x} = \frac{1}{2(1 + \sin x + \cos x)}
Cross-multiplying, we have:
2(1+sinx+cosx)(1+sinx+cosx)=1+sinx+cosx+sinxcosx2(1 + \sin x + \cos x)(1 + \sin x + \cos x) = 1 + \sin x + \cos x + \sin x \cos x
2(1+sinx+cosx+sinx+sin2x+sinxcosx+cosx+sinxcosx+cos2x)=1+sinx+cosx+sinxcosx2(1 + \sin x + \cos x + \sin x + \sin^2 x + \sin x \cos x + \cos x + \sin x \cos x + \cos^2 x) = 1 + \sin x + \cos x + \sin x \cos x
2(1+2sinx+2cosx+sin2x+cos2x+2sinxcosx)=1+sinx+cosx+sinxcosx2(1 + 2\sin x + 2\cos x + \sin^2 x + \cos^2 x + 2\sin x \cos x) = 1 + \sin x + \cos x + \sin x \cos x
Since sin2x+cos2x=1\sin^2 x + \cos^2 x = 1:
2(2+2sinx+2cosx+2sinxcosx)=1+sinx+cosx+sinxcosx2(2 + 2\sin x + 2\cos x + 2\sin x \cos x) = 1 + \sin x + \cos x + \sin x \cos x
4+4sinx+4cosx+4sinxcosx=1+sinx+cosx+sinxcosx4 + 4\sin x + 4\cos x + 4\sin x \cos x = 1 + \sin x + \cos x + \sin x \cos x
3+3sinx+3cosx+3sinxcosx=03 + 3\sin x + 3\cos x + 3\sin x \cos x = 0
3(1+sinx+cosx+sinxcosx)=03(1 + \sin x + \cos x + \sin x \cos x) = 0
1+sinx+cosx+sinxcosx=01 + \sin x + \cos x + \sin x \cos x = 0
We see a mistake in the original question. The correct equation may be
cosx1+sinx+sinx1+cosx=21+sinx+cosx+sinxcosx\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{2}{1 + \sin x + \cos x + \sin x \cos x}.
Or the correct equation may be cosx1+sinx+sinx1+cosx=1+sinx+cosx1+sinx+cosx+sinxcosx\frac{\cos x}{1+\sin x} + \frac{\sin x}{1+\cos x} = \frac{1+\sin x + \cos x}{1+\sin x+\cos x + \sin x \cos x}.

3. Final Answer

The given trigonometric identity is incorrect.
Final Answer: The final answer is cosx1+sinx+sinx1+cosx=1+sinx+cosx1+sinx+cosx+sinxcosx\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{1 + \sin x + \cos x}{1 + \sin x + \cos x + \sin x \cos x}
The identity cosx1+sinx+sinx1+cosx=12(1+sinx+cosx)\frac{\cos x}{1 + \sin x} + \frac{\sin x}{1 + \cos x} = \frac{1}{2(1 + \sin x + \cos x)} is false.

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