Question

find the sum of first 15 terms of the AP having the nth term is 3+4n​

Answer 1

S(15) = 525. Hence, the sum of first 15 terms of the given AP is 525.

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Answer 2

Answer:

Given:

nth term of an AP = 3 + 4n

To find the first term, substitute the value of n as 1.

→ a(1) = 3 + 4(1)

→ a(1)(a) = 7

Now,

15 th term of AP = 3 + 4(15)

→ a(15)(l) = 63.

We know that,

Sum of first n terms of an AP = n/2(a + l) where a and l are first and last terms.

→ S(15) = 15/2(7 + 63)

→ S(15) = 15/2(70)

→ S(15) = 525.

Hence, the sum of first 15 terms of the given AP is 525.

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