Posted by kheem on Thursday, March 7, 2013 at 8:02am.
Cinema 2 has 25 seats in the first row and 30 rows in all. Each successive row contains one additional seat. How many seats are in the theater

math  Reiny, Thursday, March 7, 2013 at 10:27am
so you want
25+26+.. for 30 terms
an arithmetic series where a=25 , d=1 ,and n=30
sum(3) = (30/2)(50 + 29(1)) = 1185

math  Anonymous, Friday, July 8, 2016 at 5:33am
a1=25 d=1 n=30
sn=30/2{2(25)+(301)1}
sn=15(50+29)
sn=15(79)
sn=1,185 seats in the theatre

math  Denise, Friday, July 8, 2016 at 5:34am
a1=25 d=1 n=30
sn=30/2{2(25)+(301)1}
sn=15(50+29)
sn=15(79)
sn=1,185 seats in the theatre
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