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Analyze the function h(x) = x3 − 2x2 − 15x, finding intercepts and stationary points.

Mathematics

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Graph of h(x) = x³ − 2x² − 15x

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Key points of h(x) = x³ − 2x² − 15x
Pointxy
x-intercept−30
Maximum−1.6714.81
x-intercept00
Minimum3−36
x-intercept50

Step-by-step solution

4 steps

Let's complete the analysis for function (c) h(x)=x3−2x2−15xh(x) = x^3 - 2x^2 - 15x.

  1. Find the intercepts with the axes.

    To find the y-intercept, set x=0x=0:

    h(0)=(0)3−2(0)2−15(0)=0h(0) = (0)^3 - 2(0)^2 - 15(0) = 0

    The y-intercept is (0,0)\boxed{\text{(0,0)}}.

    To find the x-intercepts, set h(x)=0h(x)=0:

    x3−2x2−15x=0x^3 - 2x^2 - 15x = 0

    Factor out xx:

    x(x2−2x−15)=0x(x^2 - 2x - 15) = 0

    Factor the quadratic expression:

    x(x−5)(x+3)=0x(x-5)(x+3) = 0

    This gives the x-intercepts: x=0x=0, x=5x=5, and x=−3x=-3.
    The x-intercepts are (0,0), (5,0), and (-3,0)\boxed{\text{(0,0), (5,0), and (-3,0)}}.

  2. Find the stationary points.

    First, find the first derivative h′(x)h'(x):

    h′(x)=ddx(x3−2x2−15x)=3x2−4x−15h'(x) = \frac{d}{dx}(x^3 - 2x^2 - 15x) = 3x^2 - 4x - 15

    Set h′(x)=0h'(x)=0 to find the x-coordinates of the stationary points:

    3x2−4x−15=03x^2 - 4x - 15 = 0

    Using the quadratic formula x=−b±b2−4ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}:

    x=−(−4)±(−4)2−4(3)(−15)2(3)x = \frac{-(-4) \pm \sqrt{(-4)^2 - 4(3)(-15)}}{2(3)} x=4±16+1806x = \frac{4 \pm \sqrt{16 + 180}}{6} x=4±1966x = \frac{4 \pm \sqrt{196}}{6} x=4±146x = \frac{4 \pm 14}{6}

    The x-coordinates are:

    x1=4+146=186=3x_1 = \frac{4 + 14}{6} = \frac{18}{6} = 3 x2=4−146=−106=−53x_2 = \frac{4 - 14}{6} = \frac{-10}{6} = -\frac{5}{3}

    Now, find the corresponding y-coordinates by substituting these values back into h(x)h(x):
    For x=3x=3:

    h(3)=(3)3−2(3)2−15(3)=27−18−45=−36h(3) = (3)^3 - 2(3)^2 - 15(3) = 27 - 18 - 45 = -36

    Stationary point 1: (3,−36)(3, -36).

    For x=−53x=-\frac{5}{3}:

    h(−53)=(−53)3−2(−53)2−15(−53)h\left(-\frac{5}{3}\right) = \left(-\frac{5}{3}\right)^3 - 2\left(-\frac{5}{3}\right)^2 - 15\left(-\frac{5}{3}\right) h(−53)=−12527−2(259)+25h\left(-\frac{5}{3}\right) = -\frac{125}{27} - 2\left(\frac{25}{9}\right) + 25 h(−53)=−12527−509+25h\left(-\frac{5}{3}\right) = -\frac{125}{27} - \frac{50}{9} + 25 h(−53)=−125−150+67527=40027h\left(-\frac{5}{3}\right) = \frac{-125 - 150 + 675}{27} = \frac{400}{27}

    Stationary point 2: (−53,40027)\left(-\frac{5}{3}, \frac{400}{27}\right).

  3. Determine the nature of the stationary points.

    Find the second derivative h′′(x)h''(x):

    h′′(x)=ddx(3x2−4x−15)=6x−4h''(x) = \frac{d}{dx}(3x^2 - 4x - 15) = 6x - 4

    Evaluate h′′(x)h''(x) at each stationary point:
    For x=3x=3:

    h′′(3)=6(3)−4=18−4=14h''(3) = 6(3) - 4 = 18 - 4 = 14

    Since h′′(3)>0h''(3) > 0, (3,−36)(3, -36) is a local minimum.

    For x=−53x=-\frac{5}{3}:

    h′′(−53)=6(−53)−4=−10−4=−14h''\left(-\frac{5}{3}\right) = 6\left(-\frac{5}{3}\right) - 4 = -10 - 4 = -14

    Since h′′(−53)<0h''\left(-\frac{5}{3}\right) < 0, (−53,40027)\left(-\frac{5}{3}, \frac{400}{27}\right) is a local maximum.

    The stationary points are (3, -36) (local minimum) and (−53,40027) (local maximum)\boxed{\text{(3, -36) (local minimum) and } \left(-\frac{5}{3}, \frac{400}{27}\right) \text{ (local maximum)}}.

  4. Find the point of inflection.

    Set h′′(x)=0h''(x)=0:

    6x−4=06x - 4 = 0 6x=46x = 4 x=46=23x = \frac{4}{6} = \frac{2}{3}

    Now, find the corresponding y-coordinate by substituting x=23x=\frac{2}{3} into h(x)h(x):

    h(23)=(23)3−2(23)2−15(23)h\left(\frac{2}{3}\right) = \left(\frac{2}{3}\right)^3 - 2\left(\frac{2}{3}\right)^2 - 15\left(\frac{2}{3}\right) h(23)=827−2(49)−10h\left(\frac{2}{3}\right) = \frac{8}{27} - 2\left(\frac{4}{9}\right) - 10 h(23)=827−89−10h\left(\frac{2}{3}\right) = \frac{8}{27} - \frac{8}{9} - 10 h(23)=8−24−27027=−28627h\left(\frac{2}{3}\right) = \frac{8 - 24 - 270}{27} = \frac{-286}{27}

    The point of inflection is (23,−28627)\boxed{\text{} \left(\frac{2}{3}, -\frac{286}{27}\right) \text{}}.

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