Question

If the third and the $9^{th}$ terms of an A. P. be 4 and -8 respectively, find which term is zero ?

Answer 1

Here
a+2d=4
a+8d=-8

6d=-12
d=-2
so a=8

a+(n-1)d=0
8+(n-1)(-2)=0
n-1=4
n=5