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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 1 - 2008 - 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 1-2008-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 t... 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 1 - 2008 - 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, we need to calculate the derivative of the curve:

dydx=4+8x2\frac{dy}{dx} = -4 + \frac{8}{x^2}

Now, substituting x=2x = 2:

dydx=4+822=4+2=2\frac{dy}{dx} = -4 + \frac{8}{2^2} = -4 + 2 = -2

The slope of the tangent at point P is -2. Next, we find the y-coordinate of point P by substituting x=2x = 2 into the curve's equation:

y=94(2)82=984=3y = 9 - 4(2) - \frac{8}{2} = 9 - 8 - 4 = -3

Now we have the coordinates of point P as (2, -3). We can use point-slope form to write the equation of the tangent line:

yy1=m(xx1)y+3=2(x2)y=2x+43y=2x+1y - y_1 = m(x - x_1)\Rightarrow y + 3 = -2(x - 2)\Rightarrow y = -2x + 4 - 3\Rightarrow y = -2x + 1

Thus, the equation of the tangent is y=12xy = 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 tangent's slope. The slope of the tangent is -2, therefore, the slope of the normal is:

12\frac{1}{2}

Using point-slope form again, we have:

yy1=m(xx1)y+3=12(x2)y - y_1 = m(x - x_1)\Rightarrow y + 3 = \frac{1}{2}(x - 2)

Thus, y+3=12x1y=12x4y + 3 = \frac{1}{2}x - 1 \Rightarrow y = \frac{1}{2}x - 4

The equation of the normal is y=12x4y = \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 need the coordinates of points A and B.

  1. The tangent line intersects the x-axis where y=0y = 0:

    0=12x2x=1x=120 = 1 - 2x \Rightarrow 2x = 1 \Rightarrow x = \frac{1}{2}

    Thus, point A is (12,0)(\frac{1}{2}, 0).

  2. The normal line intersects the x-axis where y=0y = 0:

    0=12x412x=4x=80 = \frac{1}{2}x - 4 \Rightarrow \frac{1}{2}x = 4 \Rightarrow x = 8

    Thus, point B is (8,0)(8, 0).

Now we have the coordinates of points A (12,0)(\frac{1}{2}, 0), B (8,0)(8, 0), and P (2,3)(2, -3).

To find the area of triangle APB, we use the formula:

Area=12×base×height\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}

Base = distance between A and B: Base=812=16212=152\text{Base} = 8 - \frac{1}{2} = \frac{16}{2} - \frac{1}{2} = \frac{15}{2}

Height = y-coordinate of P = 3.

Thus, Area=12×152×3=454=11.25\text{Area} = \frac{1}{2} \times \frac{15}{2} \times 3 = \frac{45}{4} = 11.25

Therefore, the area of triangle APB is 11.25.

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