Schoolcrop

(Q21).If the 3^rd and the 9^th terms of an A.P are 4 and -8 respectively, which term of this A.P is zero?

Given: An A.P. whose

3^rd term = a + d = ......(1)

9^th term = a + d = -.......(2)

-d =

⇒ d =

-

= -

Substituting d = - in equation (1) we get

a + × (-) =

⇒ a – =

⇒ a = + =

Let n^th term of the given A.P be equal to zero.

a_n = a + (n – 1)d

⇒ = + (n – 1) × (-)

⇒ = – n +

⇒ – n =

⇒ n = and n =

=

∴ The ^th term of the given A.P is zero.