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
Question 11
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: dxdy=−4+x28.
Next, we evaluate the derivative at the point where x=2. dxdyx=2=−4+228=−4+2=−2.
The slope of the tangent at point P is -2.
Now we find the y-coordinate of point P: y=9−4(2)−28=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(x−2)
which simplifies to: y+3=−2x+4⇒y=−2x+1.
Hence, we confirm that the equation of the tangent at P is: 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=21.
Using point-slope form again at point P (2, -3): y−(−3)=21(x−2)
which simplifies to: y+3=21x−1⇒y=21x−4.
Thus, the equation of the normal at P is: y=21x−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.
To find point A, where the tangent intersects the x-axis (where y=0):
0=1−2x⇒2x=1⇒x=21.
Thus, point A is (0.5, 0).
To find point B, where the normal intersects the x-axis:
0=21x−4⇒21x=4⇒x=8.
Thus, point B is (8, 0).
Now, we can calculate the area of triangle APB using the formula: Area=21×base×height.
The base AB is the distance between points A and B, which is 8−0.5=7.5. The height is the y-coordinate of point P, which is -3.
Thus, the area is: Area=21×7.5×3=222.5=11.25.
Therefore, the area of triangle APB is 11.25.