[b]a)[/b] Show that the expression $ x^3-5x^2+8x-4 $ is nonegative, for every $ x\in [1,\infty ) . $

[b]b)[/b] Determine $ \min_{a,b\in [1,\infty )} \left( ab(a+b-10) +8(a+b) \right) . $ 
**Part (a):**

1. We start with the expression \( x^3 - 5x^2 + 8x - 4 \).
2. We need to show that this expression is non-negative for every \( x \in [1, \infty) \).
3. Let's factorize the expression:
   \[
   x^3 - 5x^2 + 8x - 4 = (x-1)(x-2)^2
   \]
4. To verify the factorization, we can expand the right-hand side:
   \[
   (x-1)(x-2)^2 = (x-1)(x^2 - 4x + 4) = x^3 - 4x^2 + 4x - x^2 + 4x - 4 = x^3 - 5x^2 + 8x - 4
   \]
5. Since \( (x-2)^2 \geq 0 \) for all \( x \) and \( x-1 \geq 0 \) for \( x \geq 1 \), the product \( (x-1)(x-2)^2 \geq 0 \) for \( x \in [1, \infty) \).

Thus, the expression \( x^3 - 5x^2 + 8x - 4 \) is non-negative for every \( x \in [1, \infty) \).

**Part (b):**

1. We need to determine the minimum value of \( ab(a+b-10) + 8(a+b) \) for \( a, b \in [1, \infty) \).
2. Let \( S = a + b \) and \( P = ab \). The expression becomes:
   \[
   P(S - 10) + 8S
   \]
3. Using the Arithmetic Mean-Geometric Mean (AM-GM) inequality, we have:
   \[
   S = a + b \geq 2\sqrt{ab} = 2\sqrt{P}
   \]
4. Substitute \( S \geq 2\sqrt{P} \) into the expression:
   \[
   P(S - 10) + 8S \geq P(2\sqrt{P} - 10) + 8(2\sqrt{P})
   \]
5. Simplify the expression:
   \[
   P(2\sqrt{P} - 10) + 16\sqrt{P}
   \]
6. Let \( x = \sqrt{P} \). Then \( P = x^2 \) and the expression becomes:
   \[
   x^2(2x - 10) + 16x = 2x^3 - 10x^2 + 16x
   \]
7. To find the minimum value, we take the derivative and set it to zero:
   \[
   \frac{d}{dx}(2x^3 - 10x^2 + 16x) = 6x^2 - 20x + 16
   \]
8. Solve the quadratic equation:
   \[
   6x^2 - 20x + 16 = 0 \implies 3x^2 - 10x + 8 = 0
   \]
   \[
   x = \frac{10 \pm \sqrt{100 - 96}}{6} = \frac{10 \pm 2}{6} = 2 \text{ or } \frac{4}{3}
   \]
9. Evaluate the expression at \( x = 2 \) and \( x = \frac{4}{3} \):
   \[
   \text{At } x = 2: 2(2^3) - 10(2^2) + 16(2) = 16 - 40 + 32 = 8
   \]
   \[
   \text{At } x = \frac{4}{3}: 2\left(\frac{4}{3}\right)^3 - 10\left(\frac{4}{3}\right)^2 + 16\left(\frac{4}{3}\right) = \frac{128}{27} - \frac{160}{9} + \frac{64}{3} = \frac{128 - 480 + 576}{27} = \frac{224}{27} \approx 8.296
   \]
10. The minimum value is at \( x = 2 \), giving:
    \[
    \min_{a,b \in [1, \infty)} \left( ab(a+b-10) + 8(a+b) \right) = 8
    \]

The final answer is \(\boxed{8}\)