math
posted by kheem .
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

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 
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 
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