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Date November 2019 Marks available 6 Reference code 19N.2.SL.TZ0.S_7
Level Standard Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number S_7 Adapted from N/A

Question

The following table shows the probability distribution of a discrete random variable X , where  a 0 and  b 0 .

Show that  b = 0.3 a .

[1]
a.

Find the difference between the greatest possible expected value and the least possible expected value.

[6]
b.

Markscheme

correct approach  A1

eg        0.2 + 0.5 + b + a = 1 ,   0.7 + a + b = 1

b = 0.3 a        AG  N0

[1 mark]

a.

correct substitution into  E ( X )         (A1)

eg        0.2 + 4 × 0.5 + a × b + ( a + b 0.5 ) × a ,   0.2 + 2 + a × b 0.2 a

valid attempt to express E ( X ) in one variable       M1

eg        0.2 + 4 × 0.5 + a × ( 0.3 a ) + ( 0.2 ) × a ,   2.2 + 0.1 a a 2 ,

           0.2 + 4 × 0.5 + ( 0.3 b ) × b + ( 0.2 ) × ( 0.3 b ) ,    2.14 + 0.5 b b 2

correct value of greatest  E ( X )         (A1)

2.2025   (exact)

valid attempt to find least value        (M1)

eg       graph with minimum indicated,  E ( 0 )   and   E ( 0.3 )

           ( 0 2.2 )   and  ( 0.3 2.14 ) if  E ( X )  in terms of a

           ( 0 2.14 )   and  ( 0.3 2.2 ) if  E ( X )  in terms of b

correct value of least  E ( X )         (A1)

eg        2.14   (exact)

difference = 0.0625 (exact)       A1  N2

[6 marks]

b.

Examiners report

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

Syllabus sections

Topic 2—Functions » SL 2.4—Key features of graphs, intersections using technology
Show 100 related questions
Topic 4—Statistics and probability » SL 4.7—Discrete random variables
Topic 2—Functions
Topic 4—Statistics and probability

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