Question

if the nth term of an A.P is (2n +1) then the sum of its first three terms is??

Answer 1

If nth term of an A.P is 2n+1
So, it's first term is 2(1)+1 = 3
second term is 2(2)+1 = 5

So, a = 3 and d = 5-3 = 2

Now, Sum of first three term is ,

S3 = n/2[2a+(n-1)d] = 3/2[2×3+(3-1)2]
or S3 = 3/2[6+4]
so,S3 = 15 Ans

Answer 2

Answer:

Step-by-step explanation:

If the nth terms are taken from the natural numbers then

(2(1)+1)= 3

(2(2)+1)=5

(2(3)+1)=7

Hence , ä(first term)=3 ,dif= 2,n=3

S3= 3/2 (2x3+(3-1)2)

=3/2(6+4) => 3/2(10)

= 3(5)

=> ((s3 = 15))

Hence the sum of first three terms is 15.