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Date November 2020 Marks available 2 Reference code 20N.2.SL.TZ0.S_4
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number S_4 Adapted from N/A

Question

Let fx=4-x3 and gx=lnx, for x>0.

Find fgx.

[2]
a.

Solve the equation fgx=x.

[2]
b.i.

Hence or otherwise, given that g2a=f-12a, find the value of a.

[3]
b.ii.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

attempt to form composite (in any order)       (M1)

eg    flnx , g4-x3

fgx=4-lnx3      A1  N2

[2 marks]

a.

valid approach using GDC      (M1)

eg      , 2.85, 2.85

2.85056

2.85      A1  N2

[2 marks]

b.i.

METHOD 1 – (using properties of functions)

recognizing inverse relationship       (M1)

eg     fg2a=ff-12a  =2a

equating 2a to their x from (i)       (A1)

eg     2a=2.85056

1.42528

a=1.43       A1  N2

 

METHOD 2 – (finding inverse)

interchanging x and y (seen anywhere)       (M1)

eg     x=4-y3 , f-1x=4-x3

correct working       (A1)

eg     4-2a3=ln2a, sketch showing intersection of f-12x and g2x

1.42528

a=1.43       A1  N2

 

[3 marks]

b.ii.

Examiners report

[N/A]
a.
[N/A]
b.i.
[N/A]
b.ii.

Syllabus sections

Topic 2—Functions » SL 2.5—Modelling functions
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Topic 2—Functions

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