Here’s a step-by-step approach to simplifying the expressions in the provided problems:

### Câu 1:
#### a) Rút gọn A:
- Given:
\[
A = \frac{\sqrt{x}}{\sqrt{x}+5} + \frac{2}{\sqrt{x}} - \frac{10 - 8\sqrt{x}}{x + 5\sqrt{x}}
\]
- Start by finding a common denominator for the terms and combine them.

#### b) Tìm giá trị của A khi \( x^2 = 81 \):
- Substitute \( x = 9 \) into the simplified expression for \( A \).

#### c) Tìm x để A < \( \frac{7}{4} \):
- Solve the inequality based on the simplified form of \( A \).

### Câu 2:
#### a) Rút gọn A:
- Given:
\[
A = \frac{\sqrt{x}+5}{\sqrt{x}-2}
\]
#### b) Để rút gọn B:
- Given:
\[
B = \frac{\sqrt{x}+2}{\sqrt{x}-2} + \frac{4}{\sqrt{x}-2} + \frac{8}{4\cdot x}
\]
- Follow similar steps of finding a common denominator.

### Câu 3:
#### a) \(\sqrt{3x-8} = q\):
- Isolate \( x \) and solve the equation.

#### b) \( 5 - \sqrt{6-3x} = 15 \):
- Rearrange and isolate the square root, then square both sides to find \( x \).

#### c) \( \sqrt{4x - 4} = 16 \):
- Again, isolate \( x \) after manipulating the equation.

#### d) \( \sqrt{2x+3} - \sqrt{18x+27} = -4 \):
- Combine like terms, isolate square roots, and square both sides.

For detailed calculation, follow algebraic methods and ensure accuracy with each step. If you need specific calculations for any part, let me know!