Question

Which term of the ap :
21,18,15.... Is -81

Answer 1

a=21

d= -3

an= -81

an=a+(n-1)d

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

-81=21-3n+3

-81-21=-3n+3

-102-3=-3n

-105=-3n

n=-105/-3

n=35

Answer 2

n=35 this is the final answer,hope it would be helpful