Question

if an= 2n + 3 then the value of S3 is
a)15
b)17
c)21
d)24
​

Answer 1

Answer:

21(option C)

Step-by-step explanation:

an=2n+3

a(1)=2(1)+3

=2+3

=5

a(2)=2(2)+3

= 4+3

=7

a(3)=2(3)+3

=6+3

=9

So,Ap are 5,7,9

Sum of these 3 numbers are =5+7+9

=21

Answer 2

The value of S3 will be 21 (option c).

Given,

an = 2n+3

To find,

The value of S3.

Solution,

The value of S3 will be 21.

We can easily solve this problem by following the given steps.

According to the question,

an = 2n+3 where n is the number of the term.

We know that the formula to find the sum of n terms is given as follows:

Sn = n/2(a+an) where an is the last term and a is the first term of the AP.

So, we have to find the value of a and a3.

an = 2n+3

a = 2(1)+3

a = 2+3

a = 5

a3 = 2(3)+3

a3 = 6+3

a3 = 9

Now,

S3 = 3/2 (5+9)

S3 = (3×14)/2

S3 = 42/2

S3 = 21

Hence, the value of S3 is 21.