Given A.P: 121, 117, 113, ……

a , a₁ = and d = a₂ – a₁ = – = -

Let the n^th term be the first negative term of the given G.P.

Then, a_n < 0

⇒ a_n = a + (n – 1) d < 0

⇒ + (n – 1) (-) < 0

⇒ – n + < 0

⇒ – n < 0

⇒ -n < -

⇒ n >

n >

∴ When n = , the term becomes negative, (or) nd term is the first negative term of the given A.P.