Find the values of `a` for which all the roots of the euation `x^4-4x^3-8x^2+a=0` are real.

Explanation

To find the values of \( a \) for which all the roots of the equation \( x^4 - 4x^3 - 8x^2 + a = 0 \) are real, we can follow these steps:

### Step 1: Define the function
Let \( f(x) = x^4 - 4x^3 - 8x^2 + a \).

### Step 2: Find the derivative
We need to find the derivative of \( f(x) \):
\[
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