SOLUTION: "Find the sum of the first 50 terms of the arithmetic series with first term = -1 and fifth term 9." So far I tried to find d. Then I tried to plug it into a(sub n) = a(sub 1) +

Algebra ->  Algebra  -> Sequences-and-series -> SOLUTION: "Find the sum of the first 50 terms of the arithmetic series with first term = -1 and fifth term 9." So far I tried to find d. Then I tried to plug it into a(sub n) = a(sub 1) +      Log On

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Question 81308: "Find the sum of the first 50 terms of the arithmetic series with first term = -1 and fifth term 9."
So far I tried to find d. Then I tried to plug it into a(sub n) = a(sub 1) + (n - 1)d, so that I could find a(sub n). Then I plugged everything into the formula S(sub n) = n/2(a(sub 1) + a(sub n). But I got a number that doesn't look right. If anyone could help me with this, I would very much appreciate it. Thanks.
Answer by stanbon(48562) About Me  (Show Source):
You can put this solution on YOUR website!
"Find the sum of the first 50 terms of the arithmetic series with first term =
-1 and fifth term 9."
------------
a(1) = -1
a(5) = -1 + 4d = 9
4d=9+1
d=5/2=2.5
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a(50) = -1+49d=-1+49*2.5= 121.5
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S(50) = (50/2)(-1+121.5)
=25(120.5)
=3012.5
==========
Cheers,
Stan H.