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Date May 2018 Marks available 4 Reference code 18M.3srg.hl.TZ0.3
Level HL only Paper Paper 3 Sets, relations and groups Time zone TZ0
Command term Show Question number 3 Adapted from N/A

Question

The relation R is defined such that xRy if and only if |x|+|y|=|x+y| for xy, yR.

Show that R is reflexive.

[2]
a.i.

Show that R is symmetric.

[2]
a.ii.

Show, by means of an example, that R is not transitive.

[4]
b.

Markscheme

(for xR), |x|+|x|=2|x|    A1

and |x|+|x|=|2x|=2|x|    A1

hence xRx

so R is reflexive    AG

Note: Award A1 for correct verification of identity for x > 0; A1 for correct verification for x ≤ 0.

[2 marks]

 

a.i.

if xRy|x|+|y|=|x+y|

|x|+|y|=|y|+|x|    A1

|x+y|=|y+x|     A1

hence yRx

so R is symmetric      AG

[2 marks]

a.ii.

recognising a condition where transitivity does not hold     (M1)

(egx > 0, y = 0 and z < 0)

for example, 1R0 and 0R(−1)    A1

however |1|+|1||1+1|      A1

so 1R(−1) (for example) is not true      R1

hence R is not transitive       AG

[4 marks]

b.

Examiners report

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

Syllabus sections

Topic 8 - Option: Sets, relations and groups » 8.2 » Ordered pairs: the Cartesian product of two sets.

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