Question

How many terms of AP 21,18,15....must be added to get sum 0.

Answer 1

hope it helps..........
was it the brainliest????

Answer 2

Answer:

$Sum\:of \: 15\: terms =0$

Step-by-step explanation:

Given A.P :

21,18,15,...

First term (a)=21,

$Common\: difference (d)\\=a_{2}-a_{1}\\=18-21\\=-3$

Let Sum of n terms = 0

$S_{n}=0$

$\implies \frac{n}{2}[2a+(n-1)d]=0$

$\implies 2a+(n-1)d=0$

$\implies 2\times 21+(n-1)(-3)=0$

$\implies 42-3n+3=0$

$\implies 45-3n=0$

$\implies -3n=-45$

$\implies n =\frac{-45}{-3}$

$\implies n = 15$

Therefore,

$Sum\:of \: 15\: terms =0$

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