Question

If the fifth term of ap is 5.then the sum of first 9 terms is

Answer 1

it has given that the fifth term of an ap is 5.

To find : The sum of first 9 terms is ......

solution : we know nth term of an ap is given by, Tn = a + (n - 1)d

so, 5th term , T₅ = a + (5 - 1)d = a + 4d = 5 .....(1)

now sum of n terms of an ap is given by,

Sn = n/2 [2a + (n - 1)d ]

so, sum of first 9 terms = 9/2 [2a + (9 - 1)d]

= 9/2 [2a + 8d ]

= 9 (a + 4d)

from equation (1) we get,

= 9 × 5

= 45

Therefore the sum of 9 term of the ap is 45.