SOLUTION: Find a formula for Sn the sum of the first n terms of the sequence 25,-5,5......

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Question 882108: Find a formula for Sn the sum of the first n terms of the sequence 25,-5,5......
Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
This is neither arithmetic nor geometric.
25 -30 40
-5 10 40
5 50
55
The fourth term is 55
Term n =Tn
Tn=20*x^2-90*x+95
1 25 25
2 -5 20
3 5 25
4 55 80
5 145 225
6 275 500
7 445 945
8 655 1600
9 905 2505
10 1195 3700
11 1525 5225
12 1895 7120
13 2305 9425
14 2755 12180
15 3245 15425
16 3775 19200
17 4345 23545
18 4955 28500
19 5605 34105
20 6295 40400