Prove the following inequality if we know that $a$ and $b$ are the legs of a right triangle , and $c$ is the length of the hypotenuse of this triangle: $$3a + 4b \le  5c.$$
When does equality holds?
1. Given that \(a\) and \(b\) are the legs of a right triangle and \(c\) is the hypotenuse, we start with the Pythagorean theorem:
   \[
   a^2 + b^2 = c^2
   \]

2. Consider the expression \((4a - 3b)^2\). Since the square of any real number is non-negative, we have:
   \[
   (4a - 3b)^2 \geq 0
   \]

3. Expanding \((4a - 3b)^2\), we get:
   \[
   (4a - 3b)^2 = 16a^2 - 24ab + 9b^2
   \]
   Therefore,
   \[
   16a^2 - 24ab + 9b^2 \geq 0
   \]

4. Using the Pythagorean identity \(a^2 + b^2 = c^2\), we can rewrite the inequality:
   \[
   16a^2 - 24ab + 9b^2 \geq 0
   \]

5. Notice that \(25(a^2 + b^2) = 25c^2\). We can compare this with the expanded form:
   \[
   25(a^2 + b^2) = 25c^2
   \]

6. Now, we need to show that:
   \[
   25c^2 \geq 16a^2 - 24ab + 9b^2
   \]

7. Since \(a^2 + b^2 = c^2\), we can substitute \(c^2\) back into the inequality:
   \[
   25c^2 \geq 16a^2 - 24ab + 9b^2
   \]

8. Dividing both sides by 25, we get:
   \[
   c^2 \geq \frac{16a^2 - 24ab + 9b^2}{25}
   \]

9. To show that \(3a + 4b \leq 5c\), we can use the fact that:
   \[
   (3a + 4b)^2 \leq (5c)^2
   \]

10. Expanding both sides, we get:
    \[
    9a^2 + 24ab + 16b^2 \leq 25c^2
    \]

11. Since \(a^2 + b^2 = c^2\), we can substitute \(c^2\) back into the inequality:
    \[
    9a^2 + 24ab + 16b^2 \leq 25(a^2 + b^2)
    \]

12. This simplifies to:
    \[
    9a^2 + 24ab + 16b^2 \leq 25a^2 + 25b^2
    \]

13. Rearranging terms, we get:
    \[
    0 \leq 16a^2 - 24ab + 9b^2
    \]

14. This is true since \((4a - 3b)^2 \geq 0\).

15. Therefore, we have shown that:
    \[
    3a + 4b \leq 5c
    \]

16. Equality holds when \((4a - 3b)^2 = 0\), which implies:
    \[
    4a = 3b \implies \frac{a}{b} = \frac{3}{4}
    \]

17. Using the Pythagorean theorem, if \(a = 3k\) and \(b = 4k\), then:
    \[
    c = \sqrt{(3k)^2 + (4k)^2} = \sqrt{9k^2 + 16k^2} = \sqrt{25k^2} = 5k
    \]

18. Thus, equality holds when \(a : b : c = 3 : 4 : 5\).

The final answer is \( \boxed{ 3a + 4b \leq 5c } \). Equality holds when \(a : b : c = 3 : 4 : 5\).