Question

in an A.P if Tn=4n+3. find S 15

Answer 1

is it An instead of Tn

Answer 2

Hi !

Here , Tn is same as the nth term ( or a n)

an = 4n + 3

a₁ = 4(1) + 3 = 7
a₂ = 4(2) + 3 = 11
a₃ = 4(3) + 3 = 15

Hence ,
The A.P is 7 ,11,15 ......

Common difference = 11 - 7 = 4
First term = a = 7

We have to find S₁₅ [ sum of 15 terms ]

so ,
no: of terms = n = 15

Sn = n/2 [ 2a + (n - 1) d]
     = 15/2 [ 2× 7 + ( 15 - 1)4 ]
     = 15/2 [ 14 + 14 × 4 ]
     = 15/2 × 70
     = 525

∴ S₁₅ = 525