Solution: 

Given that,

a₉ = 0

As we know that, to find nth term in an A.P = a + (n – 1)d

therefore, a + (9 – 1)d = 0 ⇒ a + 8d = 0 ———-(i)

Now,

29th term is given by a₂₉ = a + (29 – 1)d

=a₂₉ = a + 28d

And, a₂₉ = (a + 8d) + 20d (using (i))

= a₂₉ = 20d ———-(ii)

Similarly, 19th term is given by a₁₉ = a + (19 – 1)d

=a₁₉ = a + 18d

And, a₁₉ = (a + 8d) + 10d (using (i))

=a₁₉ = 10d ———(iii)

On comparing (ii) and (iii), we observe that

a₂₉ = 2(a₁₉)

Hence, 29th term is double the 19th term.