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, y∈R.
(b) Find the sum of the first 20 terms of {un}, giving your answer in the form a×(1+2m) where a∈C and m∈Z are to be determined.
A second sequence {vn} is defined by vn=unun+k, k∈N.
(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=|un−un+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)20−1)i (M1)
=3((2i)10−1)i (M1)
Note: Only one of the two M1s can be implied. Other algebraic methods may be seen.
=3(−210−1)i (A1)
=3i(210+1) A1
[4 marks]
(c) (i) METHOD 1
vn=(3(1+i)n−1)(3(1+i)n−1+k) M1
9(1+i)k(1+i)2n−2 A1
=9(1+i)k((1+i)2)n−1(=9(1+i)k(2i)n−1)
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)2n−2 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)n−1−3(1+i)n| M1
=3|1+i|n−1|1−(1+i)| M1
=3|1+i|n−1 A1
(=3(√2)n−1)
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)n−1|
=3|(1+i)|n−1 A1
(=3(√2)n−1)
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]