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The curve C has equation $y = x(5 - x)$ and the line L has equation $2y = 5x + 4$ - Edexcel - A-Level Maths Pure - Question 7 - 2012 - Paper 1

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The curve C has equation $y = x(5 - x)$ and the line L has equation $2y = 5x + 4$. (a) Use algebra to show that C and L do not intersect. (b) In the space on page ... show full transcript

Worked Solution & Example Answer:The curve C has equation $y = x(5 - x)$ and the line L has equation $2y = 5x + 4$ - Edexcel - A-Level Maths Pure - Question 7 - 2012 - Paper 1

Step 1

Use algebra to show that C and L do not intersect.

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Answer

To find if the curve C intersects the line L, we first express both equations in a standard form.

Starting with the equation of the curve:
y=x(5x)=5xx2y = x(5 - x) = 5x - x^2
Now, rewriting the line equation 2y=5x+42y = 5x + 4 gives us:
y=52x+2y = \frac{5}{2}x + 2

Next, we set these equations equal to each other to find the points of intersection: 5xx2=52x+25x - x^2 = \frac{5}{2}x + 2

Rearranging this leads to: x2+5x52x2=0-x^2 + 5x - \frac{5}{2}x - 2 = 0

To eliminate the fraction, we multiply through by 2:
2x2+10x5x4=0-2x^2 + 10x - 5x - 4 = 0 2x2+5x4=0-2x^2 + 5x - 4 = 0

Now we apply the quadratic formula: x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
Where a=2a = -2, b=5b = 5, and c=4c = -4.
Calculating the discriminant: D=b24ac=524(2)(4)=2532=7D = b^2 - 4ac = 5^2 - 4(-2)(-4) = 25 - 32 = -7

Since the discriminant is negative, there are no real roots, indicating that the curves do not intersect.

Step 2

In the space on page 11, sketch C and L on the same diagram, showing the coordinates of the points at which C and L meet the axes.

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Answer

Sketch of the Curves

  1. Curve C: The parabola opens downwards, and we can find its intercepts.

    • To find the x-intercepts, set y=0y = 0:
      x(5x)=0x(5 - x) = 0
      This gives us the intercepts at x=0x = 0 and x=5x = 5.
    • For the y-intercept, let x=0x = 0:
      y=0(50)=0y = 0(5 - 0) = 0
      So, (0,0)(0, 0) is the y-intercept.
  2. Line L: The line has its x-intercept when y=0y = 0:
    0=52x+20 = \frac{5}{2}x + 2
    Solving gives:
    x=45x = -\frac{4}{5}
    The y-intercept is when x=0x = 0:
    y=2y = 2, thus (0,2)(0, 2) is the y-intercept.

Marking the Axes

  • The point (5,0)(5, 0) on the x-axis for the curve C.
  • The point (0,2)(0, 2) on the y-axis for line L.
  • The point (0,0)(0, 0) for curve C on the origin.
  • Sketch the parabola downwards and a straight line passing through (0,2)(0, 2) with a positive gradient. Ensure to label the axes for clarity.

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