Date | May 2021 | Marks available | 2 | Reference code | 21M.2.SL.TZ2.3 |
Level | Standard Level | Paper | Paper 2 | Time zone | Time zone 2 |
Command term | Find | Question number | 3 | Adapted from | N/A |
Question
An arithmetic sequence has first term 60 and common difference -2.5.
Given that the kth term of the sequence is zero, find the value of k.
Let Sn denote the sum of the first n terms of the sequence.
Find the maximum value of Sn.
Markscheme
attempt to use u1+(n-1)d=0 (M1)
60-2.5(k-1)=0
k=25 A1
[2 marks]
METHOD 1
attempting to express Sn in terms of n (M1)
use of a graph or a table to attempt to find the maximum sum (M1)
=750 A1
METHOD 2
EITHER
recognizing maximum occurs at n=25 (M1)
S25=252(60+0), S25=252(2×60+24×-2.5) (A1)
OR
attempting to calculate S24 (M1)
S24=242(2×60+23×-2.5) (A1)
THEN
=750 A1
[3 marks]