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1 . 1 - 1 + 1 - 1 + 1 - 1 +.......+n সংখ্যক পদের যোগফল হবে -

A. 0
B. 1
C. [1 + (-1)n]
D. 1/2 [ 1 - (-1)n]

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Answer: Option D

Explanation:

a=1
r=−1/1=−1
Sn=a(1−r^n)/1−r
= 1{1−(−1)^n}/1−(−1)
=1−(−1)^n/2
=1/2{1−(−1)^n}