Question

Which term of the AP = 21, 18, 15, ... is -81?

Answer 1

a = 21

d = 18 - 21 = -3 .

Let -81 be the nth term.


Now,
nth term, $a_n=a+(n-1)d$

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

-81 = 21 -3n + 3

-81 = 24-3n

3n = 24 + 81
n = $\frac{105}{3}$

n=35

Therefore, the 35th term is -81.