Question

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

Answer 1

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tn = 2n+1

n=1

a = t1 = 2(1)+1

t1 = 2+1

t1 = 3

n= 2

t2 = 2(2) +1

t2 = 4+1

t2 = 5

so,

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 × 10

S3 = 3×5

S3 = 15

the sum of its first three terms is equals to 15.

Answer 2

Hope it helps...

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Or u can do it the other way too..