SOLUTION: fi8nd the sum of the arithmetic series.-4,+1+6+11+........+91

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Question 309331: fi8nd the sum of the arithmetic series.-4,+1+6+11+........+91
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
a1=-4
d=5
The sum of an arithmetic series is,
Sn=n%28%28a1%2Ban%29%2F2%29
Solve for n using the final term.
an=a1%2B%28n-1%29%2Ad=91
-4%2B%28n-1%29%285%29=91
5%28n-1%29=95
n-1=19
n=20
Then
S20=20%28%28-4%2B91%29%2F2%29
S20=870