Question

how many terms of the sequence 18 16 14 should be taken so that their sum is zero

Answer 1

$S_n=\frac{n}{2}[2a+(n-1)d]\\\Rightarrow 2\cdot18+(n-1)(-2)=0\\\Rightarrow n-1=18\\\Rightarrow n=19$

Answer 2

a=18
d= -2
s=n/2[2a+(n-1)d]=0
0=n/2[18×2+(n-1)×-2]
0=n/2 2[18-(n-1)]

0\n=(n-1)-18
18= (n-1)
n=18+1
n=19