An auditorium has 32 rows with20 seats in the first row and 6 more seats in each of the successive rows. how many seats are there in the auditorium?

Sum of a geometric sequence is

sn = (n (a1+an)) / 2

32 rows so a32 must be found

a1=20

n=32

first term is 20

common differnce is 6

term to find is 32nd

32th row has 20+6 (32-1) or206

S32 = (6 (32+206)) / 2

S32 = (3 (238))

S32=714

answer is 714 seats