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Date May 2018 Marks available 3 Reference code 18M.2.SL.TZ1.T_4
Level Standard Level Paper Paper 2 Time zone Time zone 1
Command term Find Question number T_4 Adapted from N/A

Question

Consider the function  f ( x ) = 48 x + k x 2 58 , where x > 0 and k is a constant.

The graph of the function passes through the point with coordinates (4 , 2).

P is the minimum point of the graph of f (x).

Find the value of k.

[2]
a.

Using your value of k , find f ′(x).

[3]
b.

Use your answer to part (b) to show that the minimum value of f(x) is −22 .

[3]
c.

Sketch the graph of y = f (x) for 0 < x ≤ 6 and −30 ≤ y ≤ 60.
Clearly indicate the minimum point P and the x-intercepts on your graph.

[4]
e.

Markscheme

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

48 4 + k × 4 2 58 = 2     (M1)
Note: Award (M1) for correct substitution of x = 4 and y = 2 into the function.

k = 3     (A1) (G2)

[2 marks]

a.

48 x 2 + 6 x      (A1)(A1)(A1)(ft) (G3)

Note: Award (A1) for −48 , (A1) for x−2, (A1)(ft) for their 6x. Follow through from part (a). Award at most (A1)(A1)(A0) if additional terms are seen.

[3 marks]

b.

48 x 2 + 6 x = 0      (M1)

Note: Award (M1) for equating their part (b) to zero.

x = 2     (A1)(ft)

Note: Follow through from part (b). Award (M1)(A1) for  48 ( 2 ) 2 + 6 ( 2 ) = 0 seen.

Award (M0)(A0) for x = 2 seen either from a graphical method or without working.

48 2 + 3 × 2 2 58 ( = 22 )    (M1)

Note: Award (M1) for substituting their 2 into their function, but only if the final answer is −22. Substitution of the known result invalidates the process; award (M0)(A0)(M0).

−22     (AG)

[3 marks]

c.

(A1)(A1)(ft)(A1)(ft)(A1)(ft)

Note: Award (A1) for correct window. Axes must be labelled.
(A1)(ft) for a smooth curve with correct shape and zeros in approximately correct positions relative to each other.
(A1)(ft) for point P indicated in approximately the correct position. Follow through from their x-coordinate in part (c). (A1)(ft) for two x-intercepts identified on the graph and curve reflecting asymptotic properties.

[4 marks]

e.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.
[N/A]
e.

Syllabus sections

Topic 2—Functions » SL 2.2—Functions, notation domain, range and inverse as reflection
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