Question #338887

For what values of 𝑐 does the curve 𝑓(𝑥) = 2𝑥^3 + 𝑐𝑥^2 + 2𝑥 have maximum and minimum points?




Show that the minimum and maximum points of every curve in the family of polynomials


𝑓(𝑥) = 2𝑥^3 + 𝑐𝑥^2 + 2𝑥 lie on the curve 𝑦 = 𝑥 − 𝑥^3.




1
Expert's answer
2022-05-09T18:45:56-0400

We point out that the function increases for large values of x|x|. It is due to the fact that the term 2x32x^3 increases as xx increases. Consider the derivative: f=6x2+2cx+2f'=6x^2+2cx+2. In case we consider the equation f=0f'=0, we get the following discriminant: D=4c248=4(c212)D=4c^2-48=4(c^2-12). Thus, for c12|c|\leq\sqrt{12}, f0f'\geq0. It means, that for those values of cc the function f(x)f(x) increases for all values of xx. Thus, minimum and maximum points are absent. For c>12|c|>\sqrt{12} we get: x1=2cD12x_1=\frac{-2c-\sqrt{D}}{12}, x2=2c+D12x_2=\frac{-2c+\sqrt{D}}{12}. The function ff increases for x(,x1)x\in(-\infty,x_1) and for (x2.+)(x_2.+\infty). It decreases for x(x1,x2)x\in(x_1,x_2). Thus, x1x_1 is a local maximum and x2x_2 is a local minimum. Consider the points of intersection of the functions f(x)=2x3+cx2+2xf(x)=2x^3+cx^2+2x, y=xx3y=x-x^3. We receive: 3x3+cx2+x=03x^3+cx^2+x=0. As we can see, the equation is the same as the equation f=0f'=0. It means that minimum and maximum points can be found from intersection of curves f(x)=2x3+cx2+2xf(x)=2x^3+cx^2+2x and y=xx3y=x-x^3.

Answer: In case c12|c|\leq\sqrt{12}, function f(x)f(x) has no minimum and maximum points. For c>12|c|>\sqrt{12}, x1=2cD12x_1=\frac{-2c-\sqrt{D}}{12}, where D=4c248=4(c212)D=4c^2-48=4(c^2-12), is a local maximum and x2=2c+D12x_2=\frac{-2c+\sqrt{D}}{12} is a local minimum.


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