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Date November 2021 Marks available 2 Reference code 21N.1.AHL.TZ0.6
Level Additional Higher Level Paper Paper 1 Time zone Time zone 0
Command term Find Question number 6 Adapted from N/A

Question

An infinite geometric sequence, with terms un, is such that u1=2 and Σk=1uk=10.

Find the common ratio, r, for the sequence.

[2]
a.

Find the least value of n such that un<12.

[3]
b.

Markscheme

10=21-r          (M1)

r=0.8               A1


[2 marks]

a.

2×0.8n-1<0.5  OR  2×0.8n-1=0.5             (M1)

n> 7.212             (A1)

n=8               A1


Note: If n=7 is seen, with or without seeing the value 7.212 then award M1A1A0.


[3 marks]

b.

Examiners report

A number of candidates did not attempt what should have been a straightforward question. Perhaps because it relied on a part of the syllabus that is restricted to HL and is not in common with SL. Some attempted it but were unaware of the formula for the sum of an infinite geometric sequence, although this is in the formula booklet. By far the biggest error was to fail to recognize that n was the smallest integer value greater than that found from solving the equation. There were disappointingly few candidates who adopted a tabular or graphical approach to this question using technology. Some relied on trial-and-error.

a.

A number of candidates did not attempt what should have been a straightforward question. Perhaps because it relied on a part of the syllabus that is restricted to HL and is not in common with SL. Some attempted it but were unaware of the formula for the sum of an infinite geometric sequence, although this is in the formula booklet. By far the biggest error was to fail to recognize that n was the smallest integer value greater than that found from solving the equation. There were disappointingly few candidates who adopted a tabular or graphical approach to this question using technology. Some relied on trial-and-error.

b.

Syllabus sections

Topic 1—Number and algebra » SL 1.3—Geometric sequences and series
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Topic 1—Number and algebra

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