Question

in a flower bed there are 23 in first row ,21 in second row , 19 in third row and so on. there are 5 plants in last row.hiw many rows are there?

Answer 1

Here a=23 ; d= -2 and an=5

an = a + (n-1)d

-->5 = 23 + (n-1)-2

-->5 = 23 +2 -2n

-->5 = 25 -2n

--> -2n = -20

-->n=10

Sn = n/2(a + an)

--> 10/2(23 + 5)

-->5 X 28

=140

-->There are 140 roses on the flower bed..