Question

How many terms of an arithmetic sequence 25, 19, 13.... add up to 20?


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Answer 1

Answer:

First find the 40th term:

a40=a1+(n−1)d        =2+39(3)=119

Then find the sum:

Sn=n(a1 + an)2S40=40(2 + 119)2=2420

Answer 2

Answer:

To find the number of terms in an arithmetic sequence, divide the common difference into the difference between the last and first terms, and then add 1.

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