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Date May 2019 Marks available 7 Reference code 19M.2.AHL.TZ2.H_7
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Prove Question number H_7 Adapted from N/A

Question

Suppose that u1u1 is the first term of a geometric series with common ratio rr.

Prove, by mathematical induction, that the sum of the first nn terms, snsn is given by

sn=u1(1rn)1rsn=u1(1rn)1r, where nZ+.

Markscheme

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

n=1s1=u1, so true for n=1              R1

assume true for n=k, ie. sk=u1(1rk)1r              M1

Note: Award M0 for statements such as “let n=k”.

Note: Subsequent marks after the first M1 are independent of this mark and can be awarded.

sk+1=sk+u1rk              M1

sk+1=u1(1rk)1r+u1rk         A1

sk+1=u1(1rk)1r+u1rk(1r)1r

sk+1=u1u1rk+u1rkru1rk1r         A1

sk+1=u1(1rk+1)1r         A1

true for n=1 and if true for n=k then true for n=k+1, the statement is true for any positive integer (or equivalent).        R1

Note: Award the final R1 mark provided at least four of the previous marks are gained.

[7 marks]

Examiners report

[N/A]

Syllabus sections

Topic 1—Number and algebra » SL 1.2—Arithmetic sequences and series
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Topic 1—Number and algebra

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