Find the sum of the first 18 terms of the arithmetic sequence whose nth term is an = 3n - 1.

495

Step-by-step explanation:

In order to find the sum of the first 18 terms you have to find the 18th term and the first term using the equation given.

a1=3(1)-1        a1=2

a18=3(18)-1   a18= 53

Then plug in 53 for an, 18 for n, and 2 in for a1 in the sum equation: Sn=n/2(a1+an)  

Sn=18/2(2+53)  Solve for sn= 495

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