SOLUTION: Find S20 for the arithmetic series 24+16+8 + … .

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Question 1160228: Find S20 for the arithmetic series 24+16+8 + … .
Found 2 solutions by Boreal, MathTherapy:
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
sum is (n/2)(2a+d(n-1))
a1=24
a2=24-8, and d=8, n=20
a3=24-(8*2)=8
a20=24-(8*19)=-128
sum is (10)(48+(-8)(19))=10 (-104)=-1040
check
sum
24/16/8/0/-8/-16/...../-128

Answer by MathTherapy(10552) About Me  (Show Source):
You can put this solution on YOUR website!

Find S20 for the arithmetic series 24+16+8 + … .
Formula for the SUM of an A.P.: matrix%281%2C3%2C+S%5Bn%5D%2C+%22=%22%2C+%28n%2F2%29%282a%5B1%5D+%2B+%28n+-+1%29d%29%29

Having being given 

matrix%281%2C3%2C+S%5Bn%5D%2C+%22=%22%2C+%28n%2F2%29%282a%5B1%5D+%2B+%28n+-+1%29d%29%29 becomes: