MBA aspirants who are appearing in CMAT Test which will be conducted in January 2022 may find following Quantitative Techniques
Question useful for practice:
Progressions: Sum of terms of an Arithmetic Sequences
Question
The sum of 20 terms of the series -12 + 22 - 32 + 42 - 52 + 62....
1.210
2.519
3.190
4.420
Answer
Correct Choice is (1) and Correct Answer is 210
Explanatory Answer
-12 + 22 - 32 + 42 - 52 + 62.... up to 20 terms can be written as
(-12 + 22) + (-32 + 42) + (-52 + 62) +..... up to 20 terms.
= (-1 + 2) (1 + 2) + (-3 + 4) (3 + 4) + (-5 + 6) (5 + 6) +..... (-19 + 20) (19 + 20)
= (1 + 2) + (3 + 4) + (5 + 6) + ..... + (19 + 20)
Sum of the first 'n' natural numbers = n(n+1)/2
Therefore, sum of the given series = 20*21/2 = 210
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