Question

if the 9th term of an A.P is zero, then prove that the 29th term is double the 19th term :- this is the question

Answer 1

a + 8d = 0   ( ninth term)
a = -8d
 
19th term : a + 18d 
-8d + 18d
=10d

29th term : a + 28d
-8d + 28d = 20d
19th term is 10d     and  29th term is 20d ....
so we can say that the 29th term is double of the 18th tern

Answer 2

a+8d=0
a+28d=2 (a+18d)
a+28d = 2a+36d
a=-8d
substitute in first equation
a+8d =0
-8d +8d =0