Question

.If nth term of an A.P is (2n+1), then the sum of its first three terms is………. *​

Answer 1

Answer:

nth term of an AP is generally given by Tn=a+(n-1)d. Now here it is given that Tn=2n+1. So,

a+(n-1)d=2n+1

a+nd-d=2n+1

nd+(a-d)=2n+1

Now comparing LHS and RHS,we get

d=2 and a-d=1

a-2=1

a=3.

So now sum of first 3 terms is given by

Sn=n/2(a+a+(n-1)d)

S3=3/2*(3+3+(3–1)2)

S3=3/2*10

S3=15

Therefore sum of frist three terms is 15.

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Answer 2

Answer:

15 is your correct anwer.........