Question

which term of A.P.21,18,15,...is-81

Answer 1

A=21
D=18-21
=-3

An=a +(n-1)d
-81=21+(n-1)(-3)
-81-21=(n-1)(-3)
-102=(n-1)(-3)
-102/(-3)=n-1
34=n-1
n=34+1
n=35

So 35 term is - 81

Answer 2

Hi ,

21 , 18, 15, ....-81 is given A.P

first term = a = 21 ,

common difference ( d ) = a2 - a1

d = 18 - 21 = -3

Let nth term = -81

a + ( n - 1 )d = -81

=> 21 + ( n - 1 )( - 3 ) = -81

( n - 1 )( - 3 ) = -81 - 21

=> n - 1 = - 102/( -3 )

=> n - 1 = 34

n = 34 + 1

n = 35

Therefore ,

35th term of given A.P is - 81.

I hope this helps you.

: )