Let $r$ be the radius of the inscribed circle , $R$ be the radius of the circumscribed circle, $p$ the perimeter and $c$ the length of the hypotenuse of a right triangle.
(a) Show that $\frac{p}{c}-\frac{r}{R}=2$
(b) Of all the right triangles, what is the greatest value the ratio $\frac{r}{R}$ can take ? For which right triangles is this maximum reached?
### Part (a)
We need to show that:
\[
\frac{p}{c} - \frac{r}{R} = 2
\]

1. **Perimeter and Hypotenuse Relationship**:
   For a right triangle with legs \(a\) and \(b\) and hypotenuse \(c\), the perimeter \(p\) is given by:
   \[
   p = a + b + c
   \]

2. **Inradius \(r\)**:
   The inradius \(r\) of a right triangle is given by:
   \[
   r = \frac{a + b - c}{2}
   \]

3. **Circumradius \(R\)**:
   The circumradius \(R\) of a right triangle is given by:
   \[
   R = \frac{c}{2}
   \]

4. **Substitute \(p\), \(r\), and \(R\) into the equation**:
   \[
   \frac{p}{c} - \frac{r}{R} = \frac{a + b + c}{c} - \frac{\frac{a + b - c}{2}}{\frac{c}{2}}
   \]

5. **Simplify the expression**:
   \[
   \frac{a + b + c}{c} - \frac{a + b - c}{c} = \frac{a + b + c}{c} - \frac{a + b - c}{c}
   \]
   \[
   = \frac{a + b + c - (a + b - c)}{c} = \frac{a + b + c - a - b + c}{c} = \frac{2c}{c} = 2
   \]

Thus, we have shown that:
\[
\frac{p}{c} - \frac{r}{R} = 2
\]

### Part (b)
We need to find the maximum value of the ratio \(\frac{r}{R}\) and determine for which right triangles this maximum is reached.

1. **Express \(\frac{r}{R}\)**:
   \[
   \frac{r}{R} = \frac{\frac{a + b - c}{2}}{\frac{c}{2}} = \frac{a + b - c}{c}
   \]

2. **Use the relationship \(c = \sqrt{a^2 + b^2}\)**:
   \[
   \frac{r}{R} = \frac{a + b - \sqrt{a^2 + b^2}}{\sqrt{a^2 + b^2}}
   \]

3. **Simplify the expression**:
   \[
   \frac{r}{R} = \frac{a + b}{\sqrt{a^2 + b^2}} - 1
   \]

4. **Use the AM-GM inequality**:
   By the Arithmetic Mean-Geometric Mean (AM-GM) inequality, we have:
   \[
   \frac{a + b}{2} \geq \sqrt{ab}
   \]
   \[
   a + b \geq 2\sqrt{ab}
   \]

5. **Substitute \(a = b\)**:
   For the equality case in AM-GM, let \(a = b\):
   \[
   c = \sqrt{a^2 + a^2} = a\sqrt{2}
   \]
   \[
   \frac{r}{R} = \frac{a + a - a\sqrt{2}}{a\sqrt{2}} = \frac{2a - a\sqrt{2}}{a\sqrt{2}} = \frac{2 - \sqrt{2}}{\sqrt{2}} = \frac{2}{\sqrt{2}} - 1 = \sqrt{2} - 1
   \]

Thus, the maximum value of \(\frac{r}{R}\) is:
\[
\frac{1}{\sqrt{2} + 1}
\]

This maximum is reached for isosceles right triangles where \(a = b\).

The final answer is \( \boxed{ \frac{1}{\sqrt{2} + 1} } \)