Question

if nth term of an Ap is (3n + 5) then find the sum of first 15 term

Answer 1

Hi Friend,

Tn = 3n +5

T1 = 3 × 1 +5 => 3+5 = 8

T2= 3 × 2 +5 = 6+5 = 11

T3 = 3 × 3 +5 = 9+5 = 14

Therefore,


AP= 8,11,14.....

Here,

First term (A) = 8

Common difference (D) = 3

Sn = N/2 ×[ 2a + (n-1) × d]

S15 = 15/2 × [ 2 × 8 + (15-1) × 3]

=> 15/2 × (16+42)

=> 15/2 × 58

=> 15 × 29

=>435

Hence,

The sum of 15th term of an AP is 435

HOPE IT WILL HELP YOU.... :-)

Answer 2

1st term =3×1+5=8
2nd term =3×2+5=11
common difference=11-8=3
Sum of 15terms=15/2(2×8+(15-1)×3)
=15/2(16+42)
=15/2×58
=435