Question

The sum of the first three terms of AP whose nth term is given by an = 2n + 1 is

Answer 1

Answer:

15

Solution:

Given :

• nth term , a(n) = 2n + 1

To find :

• Sum of first three terms , S(3) = ?

We have ;

nth term , a(n) = 2n + 1

Thus,

1st term , a(1) = 2•1 + 1 = 2 + 1 = 3

2nd term , a(2) = 2•2 + 1 = 4 + 1 = 5

3rd term , a(3) = 2•3 + 1 = 6 + 1 = 7

Also ,

Sum of first n terms is given as ;

S(n) = a(1) + a(2) + a(3) + . . . + a(n)

Thus,

Sum of first three terms will be ;

=> S(3) = a(1) + a(2) + a(3)

=> S(3) = 3 + 5 + 7

=> S(3) = 15

Hence ,

Required answer is 15 .

( For alternative methods , please refer to the attachments . )