Let $(f_i)$ be a sequence of functions defined by: $f_1(x)=x, f_n(x) = \sqrt{f_{n-1}(x)}-\dfrac14$. ($n\in \mathbb{N}, n\ge2$)
(a) Prove that $f_n(x) \le f_{n-1}(x)$ for all x where both functions are defined.
(b) Find for each $n$ the points of $x$ inside the domain for which $f_n(x)=x$.
(a) We need to prove that $f_n(x) \le f_{n-1}(x)$ for all $x$ where both functions are defined.

1. Let $f_{n-1}(x) = y$. By the definition of the sequence, we have:
   \[
   f_n(x) = \sqrt{f_{n-1}(x)} - \frac{1}{4} = \sqrt{y} - \frac{1}{4}
   \]

2. We need to show that:
   \[
   \sqrt{y} - \frac{1}{4} \le y
   \]

3. Rearrange the inequality:
   \[
   \sqrt{y} - \frac{1}{4} \le y \implies \sqrt{y} \le y + \frac{1}{4}
   \]

4. Square both sides to eliminate the square root:
   \[
   (\sqrt{y})^2 \le (y + \frac{1}{4})^2 \implies y \le y^2 + \frac{1}{2}y + \frac{1}{16}
   \]

5. Rearrange the terms to form a quadratic inequality:
   \[
   0 \le y^2 + \frac{1}{2}y + \frac{1}{16} - y \implies 0 \le y^2 - \frac{1}{2}y + \frac{1}{16}
   \]

6. The quadratic expression $y^2 - \frac{1}{2}y + \frac{1}{16}$ can be rewritten as:
   \[
   y^2 - \frac{1}{2}y + \frac{1}{16} = \left(y - \frac{1}{4}\right)^2
   \]

7. Since $\left(y - \frac{1}{4}\right)^2 \ge 0$ for all $y$, the inequality holds true. Therefore, we have:
   \[
   \sqrt{y} - \frac{1}{4} \le y
   \]

8. This establishes that $f_n(x) \le f_{n-1}(x)$ for all $x$ where both functions are defined.

(b) We need to find the points $x$ inside the domain for which $f_n(x) = x$ for each $n$.

1. For $n = 1$, we have:
   \[
   f_1(x) = x
   \]

2. For $n \ge 2$, we need to solve the equation:
   \[
   f_n(x) = x \implies \sqrt{f_{n-1}(x)} - \frac{1}{4} = x
   \]

3. Since $f_n(x) = x$, we have:
   \[
   \sqrt{x} - \frac{1}{4} = x
   \]

4. Rearrange the equation:
   \[
   \sqrt{x} = x + \frac{1}{4}
   \]

5. Square both sides to eliminate the square root:
   \[
   (\sqrt{x})^2 = \left(x + \frac{1}{4}\right)^2 \implies x = x^2 + \frac{1}{2}x + \frac{1}{16}
   \]

6. Rearrange the terms to form a quadratic equation:
   \[
   x^2 + \frac{1}{2}x + \frac{1}{16} - x = 0 \implies x^2 - \frac{1}{2}x + \frac{1}{16} = 0
   \]

7. Solve the quadratic equation using the quadratic formula $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$, where $a = 1$, $b = -\frac{1}{2}$, and $c = \frac{1}{16}$:
   \[
   x = \frac{\frac{1}{2} \pm \sqrt{\left(\frac{1}{2}\right)^2 - 4 \cdot 1 \cdot \frac{1}{16}}}{2 \cdot 1} = \frac{\frac{1}{2} \pm \sqrt{\frac{1}{4} - \frac{1}{4}}}{2} = \frac{\frac{1}{2} \pm 0}{2} = \frac{1}{4}
   \]

8. Therefore, the solution to the equation is:
   \[
   x = \frac{1}{4}
   \]

The final answer is $\boxed{\frac{1}{4}}$