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The line $y = x + 2$ meets the curve $x^2 + 4y^2 - 2x = 35$ at the points A and B as shown in Figure 2 - Edexcel - A-Level Maths Pure - Question 2 - 2013 - Paper 3

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The-line-$y-=-x-+-2$-meets-the-curve-$x^2-+-4y^2---2x-=-35$-at-the-points-A-and-B-as-shown-in-Figure-2-Edexcel-A-Level Maths Pure-Question 2-2013-Paper 3.png

The line $y = x + 2$ meets the curve $x^2 + 4y^2 - 2x = 35$ at the points A and B as shown in Figure 2. (a) Find the coordinates of A and the coordinates of B. (b)... show full transcript

Worked Solution & Example Answer:The line $y = x + 2$ meets the curve $x^2 + 4y^2 - 2x = 35$ at the points A and B as shown in Figure 2 - Edexcel - A-Level Maths Pure - Question 2 - 2013 - Paper 3

Step 1

Find the coordinates of A and the coordinates of B.

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Answer

To find the intersection points A and B of the line and the curve, we first substitute the equation of the line into the curve's equation:

  1. Substitute:

    x2+4(x+2)22x=35x^2 + 4(x + 2)^2 - 2x = 35

  2. Expand:

    x2+4(x2+4x+4)2x=35x^2 + 4(x^2 + 4x + 4) - 2x = 35

    x2+4x2+16+162x=35x^2 + 4x^2 + 16 + 16 - 2x = 35

    5x2+14x27=05x^2 + 14x - 27 = 0

  3. Solve the quadratic equation using the quadratic formula:

    x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} where a=5a = 5, b=14b = 14, c=27c = -27.

    Calculate the discriminant:

    D=14245(27)=196+540=736D = 14^2 - 4 \cdot 5 \cdot (-27) = 196 + 540 = 736

    Therefore,

    x=14±73610x = \frac{-14 \pm \sqrt{736}}{10}

    Since 736=823\sqrt{736} = 8\sqrt{23},

    x=14±82310x = \frac{-14 \pm 8\sqrt{23}}{10}

    This gives us the two x-coordinates:

  4. The coordinates of A and B are derived from:

    xA=14+82310,yA=xA+2x_A = \frac{-14 + 8\sqrt{23}}{10}, y_A = x_A + 2

    xB=1482310,yB=xB+2x_B = \frac{-14 - 8\sqrt{23}}{10}, y_B = x_B + 2

  5. Finally, we can state the coordinates of A and B as:

    Coordinates of A: (14+82310,14+82310+2)\text{Coordinates of A: } \left( \frac{-14 + 8\sqrt{23}}{10}, \frac{-14 + 8\sqrt{23}}{10} + 2 \right)
    $$\text{Coordinates of B: } \left( \frac{-14 - 8\sqrt{23}}{10}, \frac{-14 - 8\sqrt{23}}{10} + 2 \right)$

Step 2

Find the distance AB in the form $r \sqrt{2}$ where $r$ is a rational number.

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Answer

To find the distance AB, we use the distance formula between the two points A and B:

d=(xBxA)2+(yByA)2d = \sqrt{(x_B - x_A)^2 + (y_B - y_A)^2}

  1. We already know:

    yByA=(xB+2)(xA+2)=xBxAy_B - y_A = (x_B + 2) - (x_A + 2) = x_B - x_A

    Therefore, we have:

    d=(xBxA)2+(xBxA)2d = \sqrt{(x_B - x_A)^2 + (x_B - x_A)^2}

    d=2(xBxA)2=xBxA2d = \sqrt{2(x_B - x_A)^2} = |x_B - x_A| \sqrt{2}

  2. From the earlier calculations:

    xBxA=82310+82310=162310=8235x_B - x_A = \frac{8\sqrt{23}}{10} + \frac{8\sqrt{23}}{10} = \frac{16\sqrt{23}}{10} = \frac{8\sqrt{23}}{5}

  3. Thus, the distance AB is:

    d=82352d = \frac{8\sqrt{23}}{5} \sqrt{2}

    Hence, in the required form r2r \sqrt{2}, we have:

    r=8235r = \frac{8\sqrt{23}}{5}

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