Question

The sum of first 18 terms of an ap whose nth term is 3-2n is

Answer 1

Answer:

Given that nth term of an ap is 3 - 2n i.e.

$a_n = 3 - 2n$

So,

$a_1 = 3 - 2 = 1 \\ a_2 = 3 - 4 = - 1 \\ a_3 = 3 - 6 = - 3 \\ a_4 = 3 - 8 = - 5 \\ a_5 = 3 - 10 = - 7$

So, it give that

first term = a = 1

&

common difference =

d = -3.

Now , we have to find sum of its first 18 terms

So,

n=18.

$S_{18} = \frac{18}{2} (2a +17d) \\ = 9(2 - 51) \\ = 9 \times( - 49) \\ = - 441$

so sum of its first 18 terms is equals to -441 .

Answer 2

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