Question

a theatre has 14 seats in the first row 21 seats in the second row 28 seats in the third row and so on how many seats are there in the ninth row if this pattern continues in the same manner​

Answer 1

verified. answer

Answer:

First row=20, second row=24, third row=28

So, the A.P is 20,24,28

a=20

d=24−20=4

n=30

S

n

=

2

n

[2×a+(n−1)×d]

S

30

=

2

30

[2×20+(30−1)×4]

=15[40+29×4]

=15[156]

=2340

Total seats are 2340.

Answer 2

Answer:

First row =20 sec row =24 third row = 28

so, the A.P is 20,24,28

$a = 20$

$d = 24 - 20 = 4$

$n = 30$

$sn = \frac{n}{2} (2 \times a + (n - 1) \times d)$

$s30 = \frac{30}{2} (2 \times 20 + (30 - 1) \times 4)$

$= 15(40 + 29 \times 4)$

$= 15(156)$

$= 2340$

Total seats are 2340