Solution: Given, a3 = 4 and a9 = - 8
a3 = a + 2d = 4
a9 = a + 8d = - 8
Subtracting 3rd term from 9th term, we get;
a + 8d – a – 2d = - 8 – 4 = - 12
Or, 6d = - 12
Or, d = - 2
Substituting the value of d in 3rd term, we get;
a + 2(-2) = 4
Or, a – 4 = 4
Or, a = 8
Now; 0 = a + (n – 1)d
Or, 0 = 8 + (n – 1)(- 2)
Or, (n – 1)(- 2) = - 8
Or, n – 1 = 4
Or, n = 5
Thus, 5th term of this AP is zero.