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AAHL Sample Paper 3 Agnesi

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Answer all questions in the answer booklet provided. Please start each question on a new page. Full
marks are not necessarily awarded for a correct answer with no working. Answers must be supported
by working and/or explanations. Solutions found from a graphic display calculator should be supported
by suitable working. For example, if graphs are used to find a solution, you should sketch these as part
of your answer. Where an answer is incorrect, some marks may be given for a correct method, provided
this is shown by written working. You are therefore advised to show all working.

1. [Maximum mark: 29]

This question asks you to explore the derivation and properties of a curve known
as the “witch of Agnesi.”

In this question, let a be constant such that a ∈ ℤ+.


A circle centred at the point C(0, a) with radius a units is shown in the diagram
below. The point P(x, y) is positioned such that it is directly below point C and has

N
the same y-coordinate as point A. Both points A and C lie on a line passing

IO
through the origin at an angle of inclination θ, where 0 < θ < π.

AT
AM LE
IN
P
EX M
K A
S
C
O

x
(a) (i) Show that cos θ = . [2]
x 2 + 4a 2
M

(ii) Show that the equation of the line passing through OC is y = x tan θ. [2]

(iii) Using the result of part (ii), show that x = 2a cot θ. [2]

(This question continues on the following page)


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(Question 1 continued)

The y-coordinate of P is 2a sin2 θ . The point P moves as the value of θ varies.

(b) By eliminating θ, show that the equation of the path of point P is given by:
8a 3
y= [5]
x 2 + 4a 2

8a 3
(c) Show that y = is an even function. [1]
x 2 + 4a 2

(d) Find the coordinates of the points of inflexion. Give your answer in terms of a. [7]

N
8a 3
(e) Sketch the graph of y= , labelling the stationary point, points of
x 2 + 4a 2

IO
inflexion and any asymptotes. [3]

AT
AM LE
∞ R

∫k R→∞ ∫k
You may use the fact that f (x) d x = lim f (x) d x without proof.
IN
P
EX M

8a 3
(f) (i) Find the area under the curve y = for x ∈ ℝ. [5]
x 2 + 4a 2
K A
S

(ii) Describe the effect on the area under the curve if the value of a is doubled.
Justify your answer with an appropriate calculation. [2]
C
O
M

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