If 9th term of an A.P. is zero, then its 29th term is ___ its 19th term.

Let 1^st term of A.P be a and common difference be d.

now $a_9=0\Rightarrow a+8d=0\Rightarrow a=-8d\cdots \cdots \cdots \left(1\right)$

Now $a_{29}=a+28d=-8d+28d\Rightarrow a_{29}=20d\cdots \cdots \cdots \left(2\right)$

Also $a_{19}=a+18d=-8d+18d=10d$

$\Rightarrow 2\times a_{19}=2\times 10d=20d\cdots \cdots \cdots \left(3\right)$

From (2) and (3)

We have $a_{29}=2a_{19}$