Homework Help: math

Posted by Poppy on Sunday, December 18, 2011 at 7:34pm.

the ninth term of an arithmetic sequence is 23 and the 33rd term of the sequence is -241 , calculate the sume of the first 15 terms of the sequence , using the formula

s=n(2a+(n-1)d)/2

math - Reiny, Sunday, December 18, 2011 at 8:03pm
a+8d = 23
a+32d = -241
subtract them ...
24d = -264
d = -11
back into the first ....
a - 88 = 23
a = 111

now that we know a = 111, d = -11 and n = 15
just plug those values into your formula

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