The first term of an arithmetic sequence is -2 and the common difference is 5. What is the sum of the first 15 terms?
Answer and explanation
Correct answer: A. 495
- Step 1: S_n = n(2a_1 + (n - 1)d) / 2.
- Step 2: S_15 = 15(-4 + 70) / 2 = 495.
Answer
495.
Why not the others: The other values use one term too few, forget to halve, or use n times the last term.
Practise more Probability, counting and sequences.
Free questions with an explanation for every one, and full timed University of Bahrain Aptitude Test (UOB Aptitude) mock tests.
More Probability, counting and sequences questions
- A bag has 4 red and 5 blue marbles. Two marbles are drawn one after the other without replacement. What is the probability that both are red…
- 7 runners compete in a race. In how many ways can the first 2 places be awarded?
- The first term of an arithmetic sequence is 10 and the common difference is -3. What is the sum of the first 10 terms?
- What is the median of the data set 15, 5, 11, 13, 8, 12, 6?
- A bag has 3 red and 4 blue marbles. Two marbles are drawn one after the other without replacement. What is the probability that both are red…
- 6 runners compete in a race. In how many ways can the first 2 places be awarded?