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Find the sum of the first 18 terms of the serie... - JAMB Mathematics 2011 Question

Find the sum of the first 18 terms of the series 3, 6, 9,..., 36.

A
505
B
513
C
433
D
635
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Correct Option: B

3, 6, 9,..., 36.

a = 3, d = 3, i = 36, n = 18

Sn = (\frac{n}{2}) [2a + (n - 1)d

S18 = (\frac{18}{2}) [2 x 3 + (18 - 1)3]

= 9[6 + (17 x 3)]

= 9 [6 + 51] = 9(57)

= 513

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