| Date | May 2011 | Marks available | 3 | Reference code | 11M.2.sl.TZ1.3 |
| Level | SL only | Paper | 2 | Time zone | TZ1 |
| Command term | Find | Question number | 3 | Adapted from | N/A |
Question
In an arithmetic sequence u1=7u1=7 , u20=64u20=64 and un=3709un=3709 .
Find the value of the common difference.
Find the value of n .
Markscheme
evidence of choosing the formula for 20th term (M1)
e.g. u20=u1+19du20=u1+19d
correct equation A1
e.g. 64=7+19d64=7+19d , d=64−719d=64−719
d=3d=3 A1 N2
[3 marks]
correct substitution into formula for unun A1
e.g. 3709=7+3(n−1)3709=7+3(n−1) , 3709=3n+43709=3n+4
n=1235n=1235 A1 N1
[2 marks]
Examiners report
The majority of candidates gained full marks in this question. However, the presentation of work was often disappointing, and there were a few incorrect trial and error approaches in part (a) resulting in division by 20 rather than 19.
The majority of candidates gained full marks in this question. However, the presentation of work was often disappointing, and there were a few incorrect trial and error approaches in part (a) resulting in division by 20 rather than 19.
