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Date May 2022 Marks available 1 Reference code 22M.1.SL.TZ2.9
Level Standard Level Paper Paper 1 Time zone Time zone 2
Command term Hence or otherwise and Find Question number 9 Adapted from N/A

Question

The graph below shows the average savings, S thousand dollars, of a group of university graduates as a function of t, the number of years after graduating from university.

The equation of the model can be expressed in the form S=at3+bt2+ct+d, where a, b, c and d are real constants.

The graph of the model must pass through the following four points.

A negative value of S indicates that a graduate is expected to be in debt.

Write down one feature of this graph which suggests a cubic function might be appropriate to model this scenario.

[1]
a.

Write down the value of d.

[1]
b.i.

Write down three simultaneous equations for a, b and c.

[2]
b.ii.

Hence, or otherwise, find the values of a, b and c.

[1]
b.iii.

Use the model to determine the total length of time, in years, for which a graduate is expected to be in debt after graduating from university.

[3]
c.

Markscheme

Accept any one of the following (or equivalent):

one minimum and one maximum point
three x-intercepts or three roots (or zeroes)
one point of inflexion           R1


Note:
Do not accept “S shape” as a justification.

 

[1 mark]

a.

d=-5         A1

 

[1 mark]

b.i.

8=a+b+c

4=8a+4b+2c

0=27a+9b+3c            A2


Note: Award A2 if all three equations are correct.
Award A1 if at least one is correct. Award A1 for three correct equations that include the letter “d”.

 

[2 marks]

b.ii.

a=2, b=-12, c=18            A1

 

[1 mark]

b.iii.

equating found expression to zero            (M1)

0=2t3-12t2+18t-5

t=0.358216, 1.83174, 3.81003            (A1)

(so total time in debt is 3.81003-1.83174+0.358216)

2.34  2.33650 years            A1

 

[3 marks]

c.

Examiners report

Proved to be difficult with several referring to the shape of the graph, the graph increasing and decreasing, or positive and negative values fitting the context.

a.

It seemed easy to find the d-value in the function. Most candidates could derive at least one correct equation, but not always three. Many candidates did not write their equations in proper mathematical form, leaving exponents and like terms in their equations. Even those candidates who did not write correct equations in part (ii) were able to correctly find the values of a, b, and c in part (iii) using cubic regression (an off-syllabus method, but still valid and credited full marks). There were some candidates who attempted an analytic method to solve the system of equations which did not usually prove successful.

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

Some candidates realized they had to find the roots, but then did not know what to do with them. Several candidates selected one of the roots as the answer to the question, usually the largest root, clearly not understanding the relationship between the roots and the length of time in debt. Others found only one root and stated that as the answer.

c.

Syllabus sections

Topic 2—Functions » SL 2.5—Modelling functions
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