Question
The sum of the first n terms is 12 + 34 + 78 + 1516 + .......... is
Answer: Option C
:
C
The sum of the first n terms is
Sn = (1 - 12) + (1 - 122) + (1 - 123) + (1 - 124 + ........... + (1 - 12n)
= n - { 12 + 122 +............+ 12n}
= n - 12(1−12n1−12) = n - (1 - 12n) = n - 1 + 2−n.
Trick:Check for n = 1, 2 i.e., S1 = 12,S2 = 54 and
(c) ⇒ S1 = 12 and S2 = 2 + 2−2 - 1 = 54.
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:
C
The sum of the first n terms is
Sn = (1 - 12) + (1 - 122) + (1 - 123) + (1 - 124 + ........... + (1 - 12n)
= n - { 12 + 122 +............+ 12n}
= n - 12(1−12n1−12) = n - (1 - 12n) = n - 1 + 2−n.
Trick:Check for n = 1, 2 i.e., S1 = 12,S2 = 54 and
(c) ⇒ S1 = 12 and S2 = 2 + 2−2 - 1 = 54.
Was this answer helpful ?
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