A meeting Hall has 20 seats in the first row,20 seats in the second , 28 seats in the third row and so on and has in all 30 rows.How many seats are there in the meeting hall ?
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Sol. The no. of seats in each row are as follows 20, 24, 28, .........
The no. of seats in each row form an A.P. with
First term (a) = 20
Difference between the no. of seats in two successive rows (d) = 4
Total no. of rows (n) = 30
Total no. of seats in 30 rows (S30) = ?
Sn = n/2[2a + (n – 1) d]
∴ S30 = 30/2 [2 (20) + (30 – 1) 4]
∴ S30 = 15 [40 + 116]
∴ S30 = 15 (156)
∴ S30 = 2340
∴ Total no. of seats in the meeting hall is 2340.
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