CA Foundation · Quantitative Aptitude · Sequence and Series
The nth term of a sequence is given by Tn = (-1)^n × (2n + 1). What is the sum of the first 10 terms of the sequence?
The sum is 10. The terms alternate as -3, 5, -7, 9, -11, 13, -15, 17, -19, 21. Grouping consecutive terms in pairs gives 2 each time, and with five pairs the total is 5 × 2 = 10.
- A10Correct
- B-10
- C11
- D-11
Explanation
Terms: -3, +5, -7, +9, ... up to +21 for n=10. Pair them: (-3+5)=2, (-7+9)=2, and so on. There are 5 pairs, each giving 2, so the sum is 10. Taking the sign as positive for odd n would give -10, which is wrong.
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