Question

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

Answer 1

tn = 2n+1 given

then

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

now go ahead

2nd mthd....

tn=2n+1

therefore t1=2(1)+1=3

t2=2(2)+1=5

now go ahead

Answer 2

nth = 2n+1
1=2×1+1=3. ---- T1
2=2×2+1=5-----T2
d= t2 - t1
d=5-3
d=2
sn =n/2 [2a+(n-1)d]
s3= 3/2 [2×3+(3-1)2]
s3=3/2 [6+4]
s3=3/2×10
s3=15
the sum of its first three terms is 15