To find the sum of the first n terms of an arithmetic sequence use the formula,
Sn=n(a1+a2)/2 ,where n is the number of terms, a1 is the first term and an is the last term.

Find the sum of the first 40 terms of the arithmetic sequence : 2,5,8,11,14,⋯
First find the 40 th term: a40=a1+(n−1)d =2+39(3)=119
Then find the sum . 40(2+119) / 2 = 2420

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FULL DISCUSSIONHow about sum of any arithmetic series ?

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To find the sum of the first n terms of an arithmetic sequence use the formula,

Sn=n(a1+a2)/2 ,where n is the number of terms, a1 is the first term and an is the last term.

Find the sum of the first 40 terms of the arithmetic sequence : 2,5,8,11,14,⋯

First find the 40 th term: a40=a1+(n−1)d =2+39(3)=119

Then find the sum . 40(2+119) / 2 = 2420