5) \( \frac{\cos x}{\cos x-\sin x}-\frac{\cos x}{\cos x+\sin x}=\tan 2 x \)
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To solve the equation \( \frac{\cos x}{\cos x-\sin x}-\frac{\cos x}{\cos x+\sin x}=\tan 2 x \), we can start by simplifying the left-hand side. Combine the fractions: \[ \frac{\cos x}{\cos x - \sin x} - \frac{\cos x}{\cos x + \sin x} = \frac{\cos x(\cos x + \sin x) - \cos x(\cos x - \sin x)}{(\cos x - \sin x)(\cos x + \sin x)} \] Now, simplify the numerator: \[ \cos x(\cos x + \sin x) - \cos x(\cos x - \sin x) = \cos x \sin x + \cos x \sin x = 2 \cos x \sin x \] The denominator is: \[ (\cos x - \sin x)(\cos x + \sin x) = \cos^2 x - \sin^2 x = \cos 2x \] So, we can rewrite the left-hand side: \[ \frac{2 \cos x \sin x}{\cos 2x} \] Noticing that \( 2 \cos x \sin x = \sin 2x \), we have: \[ \frac{\sin 2x}{\cos 2x} = \tan 2x \] Thus, the left side simplifies to \( \tan 2x \), which matches the right side of the equation. Therefore, the equation holds true and is satisfied for all \( x \) for which \( \cos x \neq \sin x \) and \( \cos 2x \neq 0 \) (to avoid division by zero).
