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How many terms of the arithmetic progression 3, 7, 11, ... are needed to give a sum of 210?

Competition practiceAlgebra

A10
B12
C9
D11

Step-by-Step Solution

Sₙ =

n /2 [2 (3) + (n - 1) 4] = n (2 n + 1) = 210

, so

2 n^{2} + n - 210 = 0

. Solving,

n = (- 1 + 41) /4 = 10

.

Answer: A) 10

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