Explanation:

Clearly 9,7,5,... is an AP with first term a=9 and common difference d=−2 and 24,21,18,.... is an AP with first term A=24 and common difference D=−3.

Leta

n

andA

n

be the n

th

term of first and second AP respectively.

Therefore according to question,

a

n

=A

n

⇒a+(n−1)d=A+(n−1)D

⇒9+(n−1)×−2=24+(n−1)×−3

⇒−2n+11=−3n+27

⇒n=16

On substituting the value of n either in a

n

orA

n

,we get,

16

th

=9+15×−2=−21