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?

Algebra ->  Sequences-and-series -> 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?       Log On


   

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) About Me  (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.

















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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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