Question

if nth term of an ap is (2n + 1 )what is the sum of its first three terms

Answer 1

nth term =(2n+1)
putting n=1.,
1st term= (2×1+1)
1st term= 3

putting n=2 ,
2nd term = (2×2+1)
2nd term=5

common difference, d = 5-3=2
first term,a=3
so, S3=n/2{2a+(n-1)d}
3/2{2×3+(3-1)2}
3/2{6+2×2}
3/2{6+4}
3×10/2

3×5 =15 .

so, sum of three term =15..

Answer 2

Answer:

Given :

nth term = 2 n + 1

= > a₁ = 2 + 1

=  3

= > a₂ = 4 + 1

= 5

d = a₂ - a₁

= > 5 - 3

= 2

Sₙ = n / 2 [ 2 a + ( n - 1 ) d ]

S₃ = 3 / 2 [ 6 + 2 × 2 ]

S₃ = 3 / 2 [ 10 ]

S₃ = 3 × 5

S₃ = 15 .

Therefore , sum of first three numbers is 15 .