Question

the nth term of an ap is given by tn=5n-3 find the sum of 20 terms of ap

Answer 1

According to question
tn=5n-3
and we have to find sum of 20 terms of AP then
N=20 and tn=5(20)-3= 100-3
=97 Answer.

Answer 2

Given::- tn = 5n - 3

To find ::- sum of 20 terms of an AP

Solution ::-

Taking n as 1 , to find the first term.

t1 = 5(1) -3

a = 5 - 3

a = 2

Taking n as 2 , to find second term and hence find common difference.

t2 = 5(2) - 3

= 10 - 3

= 7

d = t2 - a

= 7 - 2

d = 5

The sum of first 20 terms is ::-

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

S20 = 20/2 [ 2(2) + ( 20 -1) (5)]

= 10 [ 4 + 19(5) ]

= 10 ( 4 + 95)

= 10( 99)

S20 = 990

Therefore, the sum of first 20 terms is 990

HOPE IT HELPS YOU!!