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Date May 2021 Marks available 1 Reference code 21M.2.AHL.TZ2.12
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Show that Question number 12 Adapted from N/A

Question

A function f is defined by fx=arcsinx2-1x2+1, x.

A function g is defined by gx=arcsinx2-1x2+1, x, x0.

Show that f is an even function.

[1]
a.

By considering limits, show that the graph of y=f(x) has a horizontal asymptote and state its equation.

[2]
b.

Show that f'x=2xx2x2+1 for x, x0.

[6]
c.i.

By using the expression for f'x and the result x2=x, show that f is decreasing for x<0.

 

[3]
c.ii.

Find an expression for g-1(x), justifying your answer.

[5]
d.

State the domain of g-1.

[1]
e.

Sketch the graph of y=g-1(x), clearly indicating any asymptotes with their equations and stating the values of any axes intercepts.

[3]
f.

Markscheme

EITHER

f-x=arcsin-x2-1-x2+1=arcsinx2-1x2+1=fx            R1


OR

a sketch graph of y=fx with line symmetry in the y-axis indicated            R1


THEN

so fx is an even function.            AG

 

[1 mark]

a.

as x±,  fxarcsin1π2            A1

so the horizontal asymptote is y=π2            A1 

 

[2 marks]

b.

attempting to use the quotient rule to find ddxx2-1x2+1            M1

ddxx2-1x2+1=2xx2+1-2xx2-1x2+12  =4xx2+12            A1

attempting to use the chain rule to find ddxarcsinx2-1x2+1            M1

let u=x2-1x2+1 and so y=arcsinu and dydu=11-u2

f'x=11-x2-1x2+12×4xx2+12            M1

=4xx2+12-x2-12×1x2+1            A1

=4x4x2×1x2+1            A1

=2xx2x2+1            AG

 

[6 marks]

c.i.

f'x=2xxx2+1


EITHER

for x<0, x=-x            (A1)

so f'x=-2xx2+1            A1


OR

x>0 and x2+1>0            A1

2x<0, x<0            A1


THEN

f'x<0              R1


Note:
Award R1 for stating that in f'x, the numerator is negative, and the denominator is positive.


so f is decreasing for x<0            AG


Note:
Do not accept a graphical solution

 

[3 marks]

c.ii.

x=arcsiny2-1y2+1            M1

sinx=y2-1y2+1y2sinx+sinx=y2-1            A1

y2=1+sinx1-sinx            A1

domain of g is x, x0 and so the range of g-1 must be y, y0

hence the positive root is taken (or the negative root is rejected)              R1


Note: The R1 is dependent on the above A1.


so g-1x=1+sinx1-sinx            A1


Note: The final A1 is not dependent on R1 mark.

 

[5 marks]

d.

domain is -π2x<π2            A1


Note: Accept correct alternative notations, for example, -π2, π2  or -π2, π2).
Accept [-1.57,1.57[  if correct to 3 s.f.

 

[1 mark]

e.

          A1A1A1

Note: A1 for correct domain and correct range and y-intercept at y=1
         A1 for asymptotic behaviour xπ2
         A1 for x=π2
         Coordinates are not required. 
         Do not accept x=1.57 or other inexact values.

 

[3 marks]

f.

Examiners report

[N/A]
a.
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b.
[N/A]
c.i.
[N/A]
c.ii.
[N/A]
d.
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e.
[N/A]
f.

Syllabus sections

Topic 3— Geometry and trigonometry » AHL 3.9—Reciprocal trig ratios and their pythagorean identities. Inverse circular functions
Show 44 related questions
Topic 2—Functions » AHL 2.14—Odd and even functions, self-inverse, inverse and domain restriction
Topic 2—Functions
Topic 3— Geometry and trigonometry

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