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How do you sketch the graph of y = x^3 - x?

To effectively sketch the graph of the function y=x3xy = x^3 - x, it is essential to identify key points, observe symmetry, and analyze the behavior of the graph as xx approaches large values.

1. Finding the Roots: To locate the roots, we set y=0y = 0. Solving the equation x3x=0x^3 - x = 0 yields:

x(x21)=0x(x^2 - 1) = 0

From this, we find the roots x=0x = 0, x=1x = 1, and x=1x = -1. These points represent where the graph intersects the x-axis.

2. Determining Turning Points: Next, we need to find the turning points by calculating the first derivative of the function:

y=3x21y' = 3x^2 - 1

Setting the first derivative to zero, we solve:

3x21=03x^2 - 1 = 0

This gives us the solutions:

x=±13x = \pm \frac{1}{\sqrt{3}}

To classify these turning points, we calculate the second derivative:

y=6xy'' = 6x

Evaluating the second derivative at the turning points:

  • At x=13x = \frac{1}{\sqrt{3}}:
y(13)>0y''\left(\frac{1}{\sqrt{3}}\right) > 0

This indicates a local minimum.

  • At x=13x = -\frac{1}{\sqrt{3}}:
y(13)<0y''\left(-\frac{1}{\sqrt{3}}\right) < 0

This indicates a local maximum.

3. Evaluating the Function at Turning Points: Now, we compute the values of yy at these turning points:

  • For the local minimum:
y(13)=233y\left(\frac{1}{\sqrt{3}}\right) = \frac{2}{3\sqrt{3}}
  • For the local maximum:
y(13)=233y\left(-\frac{1}{\sqrt{3}}\right) = -\frac{2}{3\sqrt{3}}

4. Analyzing Symmetry: The graph exhibits symmetry about the origin, which can be confirmed by the property y(x)=y(x)y(-x) = -y(x). This indicates that the function is odd.

5. Examining End Behavior: Finally, we analyze the end behavior of the function. As xx approaches infinity (xx \to \infty), yy also approaches infinity (yy \to \infty). Conversely, as xx approaches negative infinity (xx \to -\infty), yy approaches negative infinity (yy \to -\infty). This means the graph extends infinitely in both directions.

6. Sketching the Graph: With the roots, turning points, and symmetry established, you can proceed to plot these key points. Ensure the curve passes through the roots at (0,0)(0, 0), (1,0)(1, 0), and (1,0)(-1, 0) and follows the identified behavior, including the local maximum at (13,233)\left(-\frac{1}{\sqrt{3}}, -\frac{2}{3\sqrt{3}}\right) and the local minimum at (13,233)\left(\frac{1}{\sqrt{3}}, \frac{2}{3\sqrt{3}}\right). The final sketch will be a smooth curve that reflects all the analyzed characteristics.

Answered by: Prof. Peter Brown
IB Maths Tutor
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