SOLUTION: How many terms are in the series,
a) 26 + 20 + 14 +....+ tn if its sum is -240?
b)4 + 12 + 36 +...+ tn if its sum is 39 364?
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Question 927582: How many terms are in the series,
a) 26 + 20 + 14 +....+ tn if its sum is -240?
b)4 + 12 + 36 +...+ tn if its sum is 39 364?
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
I'll do part a) to get you started
The first term in "26 + 20 + 14 +....+ tn" is 
and the common difference is 
since we're decreasing by 6 each time.
)
(n-1))
(n)+(-6)(-1))
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)
}{2})
}{2})
)}{2})
}{2})
}{2})
}{2})
)
)
Use the quadratic formula (I'm skipping showing these steps. Let me know if you need me to show these steps) to get
Ignore any negative or fractional solutions. The only solution is 
So there are 15 terms if 
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