(I typed it wrong the 1st time) In a theater there are 15 chairs in the first row, each row has 3 more chairs than the previous
row, and there are 15 rows. How many chairs are in the theater?​​

Explanation:

We have the arithmetic sequence {15, 18, 21, ...} and we want to add up the first n = 15 terms.

The first term is a = 15 and the common difference is d = 3

Plug these values into the arithmetic sum formula below

S = 0.5*n*(2*a + d(n-1))

S = 0.5*15*(2*15 + 3*(15-1))

S = 7.5*(30 + 42)

S = 7.5*(72)

S = 540

There are 540 seats total in the theater.

The slower less efficient way to do this problem would be to list out the first 15 terms by hand like so

{15,18,21,24,27,30,33,36,39,42,45,48,51,54,57}

Then adding those fifteen terms up gives us

15+18+21+24+27+30+33+36+39+42+45+48+51+54+57 = 540

This helps confirm we have the correct answer.

assuming you mean addition: addition and subtraction are reverse operations.

step-by-step explanation: