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Date May 2018 Marks available 7 Reference code 18M.2.AHL.TZ2.H_6
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Question number H_6 Adapted from N/A

Question

Use mathematical induction to prove that (1a)n>1na for {n:nZ+,n2} where 0<a<1.

Markscheme

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

Let Pn be the statement: (1a)n>1na for some nZ+,n2 where 0<a<1 consider the case n=2:(1a)2=12a+a2     M1

>12a because a2<0. Therefore P2 is true     R1

assume Pn is true for some n=k

(1a)k>1ka     M1

Note: Assumption of truth must be present. Following marks are not dependent on this M1.

EITHER

consider (1a)k+1=(1a)(1a)k     M1

>1(k+1)a+ka2      A1

>1(k+1)aPk+1 is true (as ka2>0)     R1

OR

multiply both sides by (1a) (which is positive)      M1

(1a)k+1>(1ka)(1a)

(1a)k+1>1(k+1)a+ka2     A1

(1a)k+1>1(k+1)aPk+1 is true (as ka2>0)     R1

THEN

P2 is true Pk is true Pk+1 is true so Pn true for all n>2 (or equivalent)      R1

Note: Only award the last R1 if at least four of the previous marks are gained including the A1.

[7 marks]

Examiners report

[N/A]

Syllabus sections

Topic 1—Number and algebra » AHL 1.15—Proof by induction, contradiction, counterexamples
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Topic 1—Number and algebra

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