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The curve C has equation $y = 9 - 4x - \frac{8}{x}, \quad x > 0.$ The point P on C has x-coordinate equal to 2 - Edexcel - A-Level Maths Pure - Question 11 - 2009 - Paper 1

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The-curve-C-has-equation--$y-=-9---4x---\frac{8}{x},-\quad-x->-0.$--The-point-P-on-C-has-x-coordinate-equal-to-2-Edexcel-A-Level Maths Pure-Question 11-2009-Paper 1.png

The curve C has equation $y = 9 - 4x - \frac{8}{x}, \quad x > 0.$ The point P on C has x-coordinate equal to 2. (a) Show that the equation of the tangent to C at ... show full transcript

Worked Solution & Example Answer:The curve C has equation $y = 9 - 4x - \frac{8}{x}, \quad x > 0.$ The point P on C has x-coordinate equal to 2 - Edexcel - A-Level Maths Pure - Question 11 - 2009 - Paper 1

Step 1

Show that the equation of the tangent to C at the point P is $y = 1 - 2x$

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Answer

To find the equation of the tangent line at point P, we first calculate the derivative of the curve:
dydx=4+8x2.\frac{dy}{dx} = -4 + \frac{8}{x^2}.
Next, we evaluate the derivative at the point where x=2x = 2.
dydxx=2=4+822=4+2=2.\frac{dy}{dx}\Big|_{x=2} = -4 + \frac{8}{2^2} = -4 + 2 = -2.
The slope of the tangent at point P is -2.
Now we find the y-coordinate of point P:
y=94(2)82=984=3.y = 9 - 4(2) - \frac{8}{2} = 9 - 8 - 4 = -3.
Thus, point P is (2, -3). Using point-slope form, the equation of the tangent line can be expressed:
y(3)=2(x2)y - (-3) = -2(x - 2)
which simplifies to:
y+3=2x+4y=2x+1.y + 3 = -2x + 4 \Rightarrow y = -2x + 1.
Hence, we confirm that the equation of the tangent at P is:
y=12x.y = 1 - 2x.

Step 2

Find an equation of the normal to C at the point P.

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Answer

The gradient of the normal is the negative reciprocal of the gradient of the tangent. Since the gradient of the tangent at P is -2, the gradient of the normal is:
Gradient of normal=12.\text{Gradient of normal} = \frac{1}{2}.
Using point-slope form again at point P (2, -3):
y(3)=12(x2)y - (-3) = \frac{1}{2}(x - 2)
which simplifies to:
y+3=12x1y=12x4.y + 3 = \frac{1}{2}x - 1 \Rightarrow y = \frac{1}{2}x - 4.
Thus, the equation of the normal at P is:
y=12x4.y = \frac{1}{2}x - 4.

Step 3

Find the area of triangle APB.

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Answer

To find the area of triangle APB, we first identify the coordinates of points A and B.

  1. To find point A, where the tangent intersects the x-axis (where y=0y = 0):

    0=12x2x=1x=12.0 = 1 - 2x \Rightarrow 2x = 1 \Rightarrow x = \frac{1}{2}.

    Thus, point A is (0.5, 0).

  2. To find point B, where the normal intersects the x-axis:

    0=12x412x=4x=8.0 = \frac{1}{2}x - 4 \Rightarrow \frac{1}{2}x = 4 \Rightarrow x = 8.

    Thus, point B is (8, 0).

Now, we can calculate the area of triangle APB using the formula:
Area=12×base×height.\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}.
The base AB is the distance between points A and B, which is 80.5=7.58 - 0.5 = 7.5. The height is the y-coordinate of point P, which is -3.
Thus, the area is:
Area=12×7.5×3=22.52=11.25.\text{Area} = \frac{1}{2} \times 7.5 \times 3 = \frac{22.5}{2} = 11.25.
Therefore, the area of triangle APB is 11.25.

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