Explanation:
For the cubic function to be an increasing function, dy/dx > 0 for all real values of x.
dy/dx
= d/dx [4x³ + 4px² + 16x - 9]
= 12x² + 8px + 16.
We have 12x² + 8px + 16 > 0.
=> 3x² + 2px + 4 > 0,
where a = 3, b = 2p and c = 4.
For the quadratic function to have a range that is always positive, the discriminant must be less than 0.
(in order to have no real roots)
We have b² - 4ac < 0.
=> (2p)² - 4(3)(4) < 0
=> 4p² - 48 < 0
=> p² - 12 < 0
=> p² < 12
=> -√12 < p < √12
=> -2√3 < p < 2√3.
That was a very long process but hopefully you understand all of it!