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Find the x-intercepts, axis of symmetry, range of f(x) = -x2 - x + 6, and the equation of g(x) passing through a point.

Mathematics

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Graph of f(x) = −x² − x + 6

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Key points of f(x) = −x² − x + 6
Pointxy
x-intercept−30
Maximum−0.56.25
y-intercept06
x-intercept20

Step-by-step solution

8 steps
  1. Determine the x-intercepts of ff.

    To find the x-intercepts, set f(x)=0f(x) = 0:
    \begin{align*} -x^2 - x + 6 &= 0 \ x^2 + x - 6 &= 0 \ (x+3)(x-2) &= 0 \end{align*}
    This gives two x-intercepts:

    x=−3orx=2x = -3 \quad \text{or} \quad x = 2

    The x-intercepts are (−3;0) and (2;0)\boxed{(-3; 0) \text{ and } (2; 0)}.

  2. Write down the equation of the axis of symmetry of ff.

    The axis of symmetry for a parabola f(x)=ax2+bx+cf(x) = ax^2 + bx + c is given by the formula x=−b2ax = -\frac{b}{2a}.
    For f(x)=−x2−x+6f(x) = -x^2 - x + 6, we have a=−1a = -1 and b=−1b = -1.

    x=−−12(−1)=−−1−2=−12x = -\frac{-1}{2(-1)} = -\frac{-1}{-2} = -\frac{1}{2}

    The equation of the axis of symmetry is x=−12\boxed{x = -\frac{1}{2}}.

  3. Determine the range of ff.

    Since a=−1<0a = -1 < 0, the parabola opens downwards, meaning it has a maximum value.
    The maximum value occurs at the axis of symmetry, x=−12x = -\frac{1}{2}.
    Substitute x=−12x = -\frac{1}{2} into f(x)f(x):
    \begin{align*} f\left(-\frac{1}{2}\right) &= -\left(-\frac{1}{2}\right)^2 - \left(-\frac{1}{2}\right) + 6 \ &= -\frac{1}{4} + \frac{1}{2} + 6 \ &= -\frac{1}{4} + \frac{2}{4} + \frac{24}{4} \ &= \frac{25}{4} \end{align*}
    The maximum y-value is 254\frac{25}{4}. Since the parabola opens downwards, the range is all y-values less than or equal to this maximum.
    The range of ff is y≤254\boxed{y \le \frac{25}{4}}.

  4. Write down the equation of gg in the form g(x)=mx+cg(x) = mx + c.

    The line g(x)g(x) passes through point A(-2; 4).
    From the graph, D is the y-intercept of f(x)f(x). To find D, set x=0x=0 in f(x)f(x):

    f(0)=−(0)2−(0)+6=6f(0) = -(0)^2 - (0) + 6 = 6

    So, point D is (0; 6).
    Now, calculate the gradient mm of the line passing through A(-2; 4) and D(0; 6):

    m=y2−y1x2−x1=6−40−(−2)=22=1m = \frac{y_2 - y_1}{x_2 - x_1} = \frac{6 - 4}{0 - (-2)} = \frac{2}{2} = 1

    Since D(0; 6) is the y-intercept, c=6c=6.
    The equation of gg is g(x)=x+6\boxed{g(x) = x + 6}.

  5. Write down the average gradient between points A and D.

    The average gradient between points A and D is the slope of the line segment connecting them. Since A and D are points on the line g(x)g(x), this is simply the gradient of g(x)g(x).
    From Step 4, the gradient is m=1m=1.
    The average gradient between A and D is 1\boxed{1}.

  6. Determine the equation of hh, if hh is the reflection of ff about the x-axis and then translated 3 units to the right.

    Leave your answer in the form h(x)=a(x+p)2+qh(x) = a(x+p)^2 + q.
    First, write f(x)f(x) in the form f(x)=a(x+p)2+qf(x) = a(x+p)^2 + q. The turning point of f(x)f(x) is (−12,254)(-\frac{1}{2}, \frac{25}{4}) and a=−1a=-1.

    f(x)=−(x−(−12))2+254=−(x+12)2+254f(x) = -(x - (-\frac{1}{2}))^2 + \frac{25}{4} = -(x + \frac{1}{2})^2 + \frac{25}{4}

    Reflection about the x-axis means y→−yy \to -y, so f(x)→−f(x)f(x) \to -f(x):

    −f(x)=−(−(x+12)2+254)=(x+12)2−254-f(x) = - \left( -(x + \frac{1}{2})^2 + \frac{25}{4} \right) = (x + \frac{1}{2})^2 - \frac{25}{4}

    Translation 3 units to the right means replacing xx with (x−3)(x-3):
    \begin{align*} h(x) &= ((x-3) + \frac{1}{2})^2 - \frac{25}{4} \ &= (x - \frac{6}{2} + \frac{1}{2})^2 - \frac{25}{4} \ &= (x - \frac{5}{2})^2 - \frac{25}{4} \end{align*}
    The equation of hh is h(x)=(x−52)2−254\boxed{h(x) = (x - \frac{5}{2})^2 - \frac{25}{4}}.

  7. Write down the values of xx for which f(x)>0f(x) > 0.

    From Step 1, the x-intercepts of f(x)f(x) are x=−3x = -3 and x=2x = 2.
    Since f(x)=−x2−x+6f(x) = -x^2 - x + 6 is a parabola opening downwards, f(x)>0f(x) > 0 for the x-values between its x-intercepts.
    The values of xx for which f(x)>0f(x) > 0 are −3<x<2\boxed{-3 < x < 2}.

  8. If f(p)=f(r)=4f(p) = f(r) = 4, calculate the value of p−rp-r if r<0r < 0.

    Set f(x)=4f(x) = 4:
    \begin{align*} -x^2 - x + 6 &= 4 \ -x^2 - x + 2 &= 0 \ x^2 + x - 2 &= 0 \ (x+2)(x-1) &= 0 \end{align*}
    The solutions are x=−2x = -2 or x=1x = 1.
    We are given f(p)=4f(p) = 4 and f(r)=4f(r) = 4, so pp and rr are these two x-values.
    We are also given that r<0r < 0. Therefore, r=−2r = -2.
    This means pp must be the other value

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