CM các biểu thức sau phụ thuộc vào x

To prove the dependencies on \(x\) for the expressions you provided, we can analyze each expression step by step.

### a)
**Expression A:**
\[
A = \frac{2}{\tan x - 1} + \frac{\cot x + 1}{\cot x - 1}
\]

1. **Analyze the components:**
- \(\tan x = \frac{\sin x}{\cos x}\)
- \(\cot x = \frac{\cos x}{\sin x}\)

2. Each component depends on \(x\). Therefore, \(A\) is also dependent on \(x\).

### b)
**Expression B:**
\[
B = \sqrt{\sin^4 x + 4\cos^2 x + \sqrt{\cos^6 x + \sin^2 x}}
\]

1. Each term (\(\sin^4 x\), \(4\cos^2 x\), \(\cos^6 x\), and \(\sin^2 x\)) clearly involves \(x\).
2. Thus, \(B\) is also dependent on \(x\).

### c)
**Expression C:**
\[
C = 2\cos^4 x - \sin^4 x + \sin^2 x \cdot \cos^2 x + 3\sin^2 x
\]

1. Again, every term \((2\cos^4 x, -\sin^4 x, \sin^2 x \cdot \cos^2 x, 3\sin^2 x)\) is dependent on \(x\).
2. Consequently, \(C\) is dependent on \(x\).

### Conclusion
All three expressions \(A\), \(B\), and \(C\) depend on \(x\) through the trigonometric functions involved. Therefore, the proof is complete for each part.