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Find the sum of the first 90 terms of the sequence –4, –1, 2, 5, 8, ...

2 Answers

5 votes
This is an arithmetic sequence because there is a common difference of 3, each term is 3 greater than the previous term.

The sum of any arithmetic sequence is the average of the first and last terms times the number of terms...mathematically this winds up as:

s(n)=(2an+dn^2-dn)/2, a=initial term, n=number of terms, d=common difference

In this case:

s(n)=(-8n+3n^2-3n)/2

s(n)=(3n^2-11n)/2 so

s(90)=(3*8100-11*90)/2

s(90)=11655
User Yevgenia
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9.2k points
3 votes
im pretty sure its 11655

User Aleth
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