SOLUTION: There are 30 rows of seats in a stadium.each row had 3 more than the row in front.the last row has 120 seats.how many seats in stadium all together?

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Question 1019739: There are 30 rows of seats in a stadium.each row had 3 more than the row in front.the last row has 120 seats.how many seats in stadium all together?
Answer by tiffany222(56) About Me  (Show Source):
You can put this solution on YOUR website!
an = a1 + (n-1) d
a30 = a1 + (30-1) (3)
120 = a1 + (29) (3)
120 = a1 + 87
33 = a1

Sn = n (a1 + an) / 2
Sn = n (a1 + a30) / 2
Sn = (30) (33 + 120) /2
Sn = (30) (153) / 2
Sn = 4590 / 2
Sn = 2295

Answer: 2295 seats