Find the sum of the first 18 positive even integers.

142

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242

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342

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442

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Solution

The correct option is C $342$
Positive even integers: 2, 4, 6, 8...
$n = 18, a_{1} = 2, d = 2$
$a_{n} = a_{1} + (n - 1) d$
$a_{18} = 2 + (18 - 1) 2 = 36$
Then find the sum:
$S_{18} = \frac{n (a_{1} + a_{n})}{2}$
$S_{18} = \frac{18 (2 + 36)}{2}$
$S_{18} = \frac{684}{2}$
$S_{18} = 342$

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