math
posted by Poppy .
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+(n1)d)/2

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 
Sn=510

S15 =510