Question

If nth term of an a.p id 2n-1then the sum of its first three term is

Answer 1

$\large{\boxed{\bf{Answer}}}$

12

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Given :-

• nth term of an AP is (2n - 1)

To Find :-

• Sum of it's 1st three terms.

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$\large{\boxed{\bf{Solution}}}$

Let a be the first term and d be the common difference of this AP. Then,

a = T1 = (2 × 1 - 1) = 1

T2 = (2 × 2 - 1) = 3

∴ d = (T2 - T1) = (3 - 1) = 2

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∴ sum of first n terms :-

=> n/2 × {2a + (n - 1)d}

=> n/2 × {2 × 1 + (n - 1)3}

=> n/2 × {2 + 3n - 3}

=> n/2 × {3n - 1)

∴ sum of first three terms :-

=> 3/2 × {3 × 3 - 1)

=> 3/2 × {9 - 1}

=> 3/2 × 8

=> 3 × 4

=> 12

Hence, S3 = 12

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