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if a1 a2 a3 an 3 2n 1 2 for every n 1 then a11 equ
Question:
If a1 + a2 + a3 + .... + an = 3(2n+1 - 2 ) , for every n≥ 1, then a11 equals
CAT - 2019
CAT
Updated On:
Aug 20, 2024
6144
9144
6441
9741
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The Correct Option is
A
Solution and Explanation
If n=1 , a1 = 3(2 1+1 - 2) = 6 = 3 × 2¹
If n=2 , a1 + a2 = 3(2 2+1 - 2) = 18 ⇒ a2 = 18 - a1 = 12 = 3 × 2²
if n=3 , a1 + a2 + a3 = 3(2 3+1 - 2) = 42 ⇒ a3 = 42 - (a1 + a2) = 24 = 3 × 2³
By Following the pattern , an = 3 × 2n
∴ a11 = 3 × 2 11 = 6144
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