Question

Find the sum of 25th term of an ap which in the term is given by tn equal to 7 - 3n

Answer 1

Answer:

We have, Tn = 7 - 3n

Putting n = 1

Then, T1 = 7 - 3 × 1 = 4

Now , putting n = 2

Then, T2 = 7 - 3 × 2 = 1

So , a = 4 and d = T2 - T1 = 1 - 4 = -3

Now , we have to find the sum of first 25th term of A.P.

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

S25 = 25/2 [ 2 × 4 + ( 25 - 1 ) - 3 ]

= 25/2 [ 8 + 24 × ( - 3 ) ]

= 25/2 [ 8 - 72 ]

= 25/2 × ( - 64 )

= 25 × ( - 32 )

= - 800