| Date | November 2016 | Marks available | 4 | Reference code | 16N.1.hl.TZ0.6 |
| Level | HL only | Paper | 1 | Time zone | TZ0 |
| Command term | Prove that | Question number | 6 | Adapted from | N/A |
Question
The sum of the first n terms of a sequence {un} is given by Sn=3n2−2n, where n∈Z+.
Write down the value of u1.
[1]
a.
Find the value of u6.
[2]
b.
Prove that {un} is an arithmetic sequence, stating clearly its common difference.
[4]
c.
Markscheme
u1=1 A1
[1 mark]
a.
u6=S6−S5=31 M1A1
[2 marks]
b.
un=Sn−Sn−1 M1
=(3n2−2n)−(3(n−1)2−2(n−1))
=(3n2−2n)−(3n2−6n+3−2n+2)
=6n−5 A1
d=un+1−un R1
=6n+6−5−6n+5
=(6(n+1)−5)−(6n−5)
=6 (constant) A1
Notes: Award R1 only if candidate provides a clear argument that proves that the difference between ANY two consecutive terms of the sequence is constant. Do not accept examples involving particular terms of the sequence nor circular reasoning arguments (eg use of formulas of APs to prove that it is an AP). Last A1 is independent of R1.
[4 marks]
c.
Examiners report
[N/A]
a.
[N/A]
b.
[N/A]
c.
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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