Maximizing a in $ax^3+bx^2+cx+d$ with Constraints | POTW #424 07/06/2020

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anemone
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Here is this week's POTW:

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Given that $f(x)=ax^3+bx^2+cx+d$ and $|f'(x)|\le 1$ for $0\le x \le 1$. Find the maximum value of $a$.

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Congratulations to castor28 for his correct solution (Cool) , which you can find below:
We must fit the parabola $y=f'(x)=3x^2+2bx+c$ within the rectangle $[0,1]\times[-1,1]$.

To get the largest value of $a$ (the steepest parabola), we must have $f'(0)=f'(1)=1$, the axis at $x=\dfrac12$, and $f'\left(\dfrac12\right)=-1$:

424s.png


This gives $f'(x)= 8\left(x-\dfrac12\right)^2-1= 8x^2-8x+1$ and $\bf a=\dfrac83$.

More precisely, the condition will be satisfied for $\lvert a\rvert\le\dfrac83$.
 

1. What is the purpose of maximizing a polynomial with constraints?

The purpose of maximizing a polynomial with constraints is to find the maximum value of the polynomial while also satisfying certain conditions or limitations. This can be useful in various fields such as economics, engineering, and physics where there are constraints on the variables being studied.

2. How do you determine the constraints for a polynomial?

The constraints for a polynomial can be determined by analyzing the problem or situation at hand. These constraints could be in the form of equations, inequalities, or specific values that the variables must satisfy. It is important to clearly define the constraints before attempting to maximize the polynomial.

3. What is the general approach for maximizing a polynomial with constraints?

The general approach for maximizing a polynomial with constraints involves setting up the problem as a constrained optimization problem and then using techniques such as Lagrange multipliers or the method of substitution to find the maximum value. This typically involves finding the critical points of the polynomial and checking if they satisfy the constraints.

4. Can the maximum value of a polynomial with constraints be found analytically?

In some cases, the maximum value of a polynomial with constraints can be found analytically by solving the constrained optimization problem. However, in more complex cases, numerical methods may need to be used to approximate the maximum value.

5. How can maximizing a polynomial with constraints be applied in real-world situations?

Maximizing a polynomial with constraints can be applied in various real-world situations, such as finding the maximum profit for a company while adhering to budget constraints, determining the maximum load a bridge can withstand while considering weight limitations, or maximizing crop yield while taking into account limited resources. It can also be used in mathematical modeling and optimization problems in various fields.

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