Question

which term of the AP 21,18,15....,is zero?

Answer 1

Given

AP=21,18,15,...

Here.
a=21
d=18-21=-3

Now to find the term which is zero.

using An=a+(n-1)d
→0=21+(n-1)(-3)
→-21=(-3n)+3
→-21-3=(-3n)
→(-24)=(-3n)
→24=3n
→24/3=n
→n=8

So Zero is the 8-TH term of the AP.

Hope it Helps...
Regards,
Leukonov.