Question

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

Answer 1

Here,

$a_1=21$

$d=-3$

Let $n$ terms be added to get the sum $0.$

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

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

$42+(n-1)(-3)=0$

$42-3n+3=0$

$3n=45$

$n=15$

Hence, 15 terms of the AP must be added to get the sum 0.