Question

3.Find the sum of first 15 terms of an AP, whose nth term is 3n + 1​

Answer 1

Answer:

Given: nth term is 3-2n

To find: the sum of the first 15 terms

of an ap

Solution

• Now we have given the nth term of AP as: 3 - 2n

Lets put n = 1, we get :

a1 = 3 -2 = 1

• Lets put n = 2, we get

a2 = 3 - 4 = -1

Now from these two terms, we get common difference as:

d = -1 - 1

d = -2

Lets put n = 15, we get

a15 = 3 - 30

a15 = -27

• So the sum of 15 terms will be: n/2(a+1) Sum = (15/2)[1 +(-27)]

Sum = (15/2) x 26

Sum = -195

Answer:

-195

So the sum of 15 terms is -195

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