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Date May 2014 Marks available 17 Reference code 14M.1.hl.TZ1.13
Level HL only Paper 1 Time zone TZ1
Command term Find, Show that, and State Question number 13 Adapted from N/A

Question

A geometric sequence {un}, with complex terms, is defined by un+1=(1+i)un and u1=3.

(a)     Find the fourth term of the sequence, giving your answer in the form x+yi, x, yR.

(b)     Find the sum of the first 20 terms of {un}, giving your answer in the form a×(1+2m) where aC and mZ are to be determined.

A second sequence {vn} is defined by vn=unun+k, kN.

(c)     (i)     Show that {vn} is a geometric sequence.

          (ii)     State the first term.

          (iii)     Show that the common ratio is independent of k.

A third sequence {wn} is defined by wn=|unun+1|.

(d)     (i)     Show that {wn} is a geometric sequence.

          (ii)     State the geometrical significance of this result with reference to points on the complex plane.

Markscheme

(a)     r=1+i     (A1)

u4=3(1+i)3     M1

=6+6i     A1

[3 marks]

 

(b)     S20=((1+i)201)i     (M1)

=3((2i)101)i     (M1)

 

Note:     Only one of the two M1s can be implied. Other algebraic methods may be seen.

 

=3(2101)i     (A1)

=3i(210+1)     A1

[4 marks]

 

(c)     (i)     METHOD 1

vn=(3(1+i)n1)(3(1+i)n1+k)     M1

9(1+i)k(1+i)2n2     A1

=9(1+i)k((1+i)2)n1(=9(1+i)k(2i)n1)

this is the general term of a geometrical sequence     R1AG

 

Notes:     Do not accept the statement that the product of terms in a geometric sequence is also geometric unless justified further.

     If the final expression for vn is 9(1+i)k(1+i)2n2 award M1A1R0.

 

METHOD 2

vn+1vn=un+1un+k+1unun+k     M1

=(1+i)(1+i)     A1

this is a constant, hence sequence is geometric     R1AG

 

Note:     Do not allow methods that do not consider the general term.

 

(ii)     9(1+i)k     A1

(iii)     common ratio is (1+i)2 (=2i) (which is independent of k)     A1

[5 marks]

 

(d)     (i)     METHOD 1

wn|3(1+i)n13(1+i)n|     M1

=3|1+i|n1|1(1+i)|     M1

=3|1+i|n1     A1

(=3(2)n1)

this is the general term for a geometric sequence   R1AG

METHOD 2

wn=|un(1+i)un|     M1

=|un||i|

=|un|     A1

=|3(1+i)n1|

=3|(1+i)|n1     A1

(=3(2)n1)

this is the general term for a geometric sequence     R1AG

 

Note:     Do not allow methods that do not consider the general term.

 

(ii)     distance between successive points representing un in the complex plane forms a geometric sequence     R1

 

Note:     Various possibilities but must mention distance between successive points.

 

[5 marks]

 

Total [17 marks]

Examiners report

[N/A]

Syllabus sections

Topic 1 - Core: Algebra » 1.1 » Arithmetic sequences and series; sum of finite arithmetic series; geometric sequences and series; sum of finite and infinite geometric series.
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