Consider polynomial $f(x)={{x}^{2}}-\alpha x+1$ with $\alpha \in \mathbb{R}.$ 
a) For $\alpha =\frac{\sqrt{15}}{2}$, let write $f(x)$ as the quotient of two polynomials with nonnegative coefficients.
b) Find all value of $\alpha $ such that $f(x)$ can be written as the quotient of two polynomials with nonnegative coefficients.

Let's solve the problem step-by-step.

### Part (a)
We need to write the polynomial \( f(x) = x^2 - \frac{\sqrt{15}}{2} x + 1 \) as the quotient of two polynomials with nonnegative coefficients.

1. **Identify the polynomial:**
   \[
   f(x) = x^2 - \frac{\sqrt{15}}{2} x + 1
   \]

2. **Check if the polynomial can be written as a quotient of two polynomials with nonnegative coefficients:**
   Since \(\frac{\sqrt{15}}{2} < 2\), we can use the fact that for \(\alpha < 2\), the polynomial \(f(x)\) can be written as a quotient of two polynomials with nonnegative coefficients.

3. **Express \(f(x)\) as a quotient:**
   \[
   f(x) = \frac{x^2 - \frac{\sqrt{15}}{2} x + 1}{1}
   \]
   Here, the numerator \(x^2 - \frac{\sqrt{15}}{2} x + 1\) and the denominator \(1\) both have nonnegative coefficients.

### Part (b)
We need to find all values of \(\alpha\) such that \(f(x) = x^2 - \alpha x + 1\) can be written as the quotient of two polynomials with nonnegative coefficients.

1. **Consider the general form of the polynomial:**
   \[
   f(x) = x^2 - \alpha x + 1
   \]

2. **Determine the range of \(\alpha\):**
   For \(f(x)\) to be written as a quotient of two polynomials with nonnegative coefficients, the coefficients of the polynomial must be nonnegative or the polynomial must be expressible in such a form.

3. **Analyze the condition for nonnegative coefficients:**
   - If \(\alpha = 0\), \(f(x) = x^2 + 1\), which has nonnegative coefficients.
   - If \(0 < \alpha < 2\), the polynomial \(f(x) = x^2 - \alpha x + 1\) can be written as a quotient of two polynomials with nonnegative coefficients.
   - If \(\alpha < 0\), the polynomial \(f(x) = x^2 - \alpha x + 1\) can be written as \(\frac{x^2 - \alpha x + 1}{1}\), which has nonnegative coefficients.

4. **Combine the results:**
   Therefore, the values of \(\alpha\) such that \(f(x)\) can be written as the quotient of two polynomials with nonnegative coefficients are:
   \[
   \alpha \in (-\infty, 2)
   \]

The final answer is \( \boxed{ \alpha \in (-\infty, 2) } \)