SOLUTION: What is the sum of: S sub 21 for 23+20+17+14+... Thank you in advance!

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Question 474057: What is the sum of:

S sub 21 for 23+20+17+14+...




Thank you in advance!
Answer by Tatiana_Stebko(1539) About Me  (Show Source):
You can put this solution on YOUR website!
23, 20, 17, 14, ... an arithmetic progression
a1=23
d=a2-a1=20-23=-3
Formula Sn=%28n%2F2%29%28a1%2Ban%29, so S21=%2821%2F2%29%28a1%2Ba21%29
an=a1%2B%28n-1%29d, then a21=23%2B%2821-1%29%2A%28-3%29=23-60=-37
S21=%2821%2F2%29%2823%2B%28-37%29%29=-147