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For the graph of y = 4 + 3x − x², determine a) The coordinates of the point where the curve cuts the y-axis b) The coordinates of the point where the curve cuts the x-axis c) The coordinates of any minimum or maximum points d) Hence sketch the graph of y = 4 + 3x − x²

Mathematics

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Graph of y = 4 + 3x − x²

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Key points of y = 4 + 3x − x²
Pointxy
x-intercept−10
y-intercept04
Maximum1.56.25
x-intercept40

Step-by-step solution

10 steps

Here are the solutions to Question One.

The given curve is y=4+3x−x2y = 4 + 3x - x^2. This is a quadratic equation, representing a parabola that opens downwards (because the coefficient of x2x^2 is negative).

Question 1a) The coordinates of the point where the curve cuts the y-axis.

  1. The curve cuts the y-axis when x=0x=0.

    Substitute x=0x=0 into the equation.

    y=4+3(0)−(0)2y = 4 + 3(0) - (0)^2 y=4+0−0y = 4 + 0 - 0 y=4y = 4
  2. State the coordinates.

    The coordinates where the curve cuts the y-axis are (0,4)\boxed{(0, 4)}.

    Question 1b) The coordinates of the point where the curve cuts the x-axis.

  3. The curve cuts the x-axis when y=0y=0.

    Set the equation to 0.

    0=4+3x−x20 = 4 + 3x - x^2

    Rearrange the quadratic equation to standard form ax2+bx+c=0ax^2 + bx + c = 0:

    x2−3x−4=0x^2 - 3x - 4 = 0
  4. Factor the quadratic equation.

    We need two numbers that multiply to -4 and add to -3. These numbers are -4 and 1.

    (x−4)(x+1)=0(x-4)(x+1) = 0
  5. Solve for xx.

    x−4=0  ⟹  x=4x-4 = 0 \implies x = 4 x+1=0  ⟹  x=−1x+1 = 0 \implies x = -1
  6. State the coordinates.

    The coordinates where the curve cuts the x-axis are (−1,0) and (4,0)\boxed{(-1, 0) \text{ and } (4, 0)}.

    Question 1c) The coordinates of any minimum or maximum points.

  7. Find the derivative of the function y=4+3x−x2y = 4 + 3x - x^2 with respect to xx.

    dydx=ddx(4+3x−x2)\frac{dy}{dx} = \frac{d}{dx}(4 + 3x - x^2) dydx=0+3−2x\frac{dy}{dx} = 0 + 3 - 2x dydx=3−2x\frac{dy}{dx} = 3 - 2x
  8. Set the derivative to zero to find the x-coordinate of the stationary point.

    3−2x=03 - 2x = 0 2x=32x = 3 x=32=1.5x = \frac{3}{2} = 1.5
  9. Substitute the x-coordinate back into the original equation to find the y-coordinate.

    y=4+3(32)−(32)2y = 4 + 3\left(\frac{3}{2}\right) - \left(\frac{3}{2}\right)^2 y=4+92−94y = 4 + \frac{9}{2} - \frac{9}{4}

    To add these, find a common denominator (4):

    y=164+184−94y = \frac{16}{4} + \frac{18}{4} - \frac{9}{4} y=16+18−94y = \frac{16 + 18 - 9}{4} y=34−94y = \frac{34 - 9}{4} y=254=6.25y = \frac{25}{4} = 6.25
  10. Determine if it's a minimum or maximum point.

    The second derivative d2ydx2=ddx(3−2x)=−2\frac{d^2y}{dx^2} = \frac{d}{dx}(3-2x) = -2. Since d2ydx2<0\frac{d^2y}{dx^2} < 0, the point is a maximum.
    The coordinates of the maximum point are (32,254) or (1.5,6.25)\boxed{\left(\frac{3}{2}, \frac{25}{4}\right) \text{ or } (1.5, 6.25)}.

    Question 1d) Hence sketch the graph of y=4+3x−x2y = 4 + 3x - x^2.

    Using the points calculated:

    • y-intercept: (0,4)(0, 4)
    • x-intercepts: (−1,0)(-1, 0) and (4,0)(4, 0)
    • Maximum point: (1.5,6.25)(1.5, 6.25)

    The graph is a parabola opening downwards, passing through these points.

          ^ y
          |
    6.25  . (1.5, 6.25) Max
          |
    4     . (0, 4)
          |
          |
          |
    -1 ---.-------.-------.-----> x
          |       0       4
          |
    

    (A textual representation of the sketch is provided above. For a precise visual sketch, plot these points on a coordinate plane and draw a smooth parabolic curve through them, opening downwards.)

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