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The sum of the first n terms of the arithmetic ... - JAMB Mathematics 2009 Question

The sum of the first n terms of the arithmetic progression 5, 11, 17, 23, 29, 35, ... is
A
n(3n - 0.5)
B
n(3n + 2)
C
n(3n + 2.5)
D
n(3n + 5)
correct option: b

a = 5, d = 6, n = n

Sn = n/2(2a + (n-1)d)

= n/2(2(5) + (n-1)6)

= n/2(10 + 6n-6)

= n/2(6n+4)

= 6n2/2 + 4n/2

= 32 + 2n

= n(3n + 2)

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