SOLUTION: in an arithmetic sequence, a1=2 and a40=80. what is the value of the sum of a2+a4+a6+a8+...a40?

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Question 411231: in an arithmetic sequence, a1=2 and a40=80. what is the value of the sum of a2+a4+a6+a8+...a40?
Answer by MathLover1(20850) About Me  (Show Source):
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To find the sum of a certain number of terms of an arithmetic sequence:
Sn=n%28a1%2Ban%29%2F2
where Sn is the sum of n terms (nth partial sum), a1 is the first term, an is the nth term

given: a1=2 and a40=80
to find: the sum of a2+a4+a6+a8+...a40
Sn=n%28a1%2Ban%29%2F2

S%2840%29=40%282%2B80%29%2F2


S%2840%29=40%2A82%2F2

S%2840%29=40%2A41


S%2840%29=1640