6. (2004 National College Entrance Examination - Guangdong Paper) When $0<x<\frac{\pi}{4}$, the minimum value of the function $f(x)=\frac{\cos ^{2} x}{\cos x \sin x-\sin ^{2} x}$ is
A. $\frac{1}{4}$
B. $\frac{1}{2}$
C. 2
D. 4
6. D $f(x)=\frac{\cos ^{2} x}{\cos x \sin x-\sin ^{2} x}=\frac{1}{\tan x-\tan ^{2} x}=\frac{1}{-\left(\tan x-\frac{1}{2}\right)^{2}+\frac{1}{4}} \geqslant 4$ 

6. D $f(x)=\frac{\cos^2 x}{\cos x \sin x - \sin^2 x}=\frac{1}{\tan x - \tan^2 x}=\frac{1}{-\left(\tan x - \frac{1}{2}\right)^2 + \frac{1}{4}} \geqslant 4$