48 and 39, nth term = 93 - 9n, 21 st term = - 96
there is a difference (d) of - 9 between the terms
d = 75 - 84 = 66 - 75 = 57 - 66 = - 9
rule is : subtract 9 from each term to obtain next term in sequence
This is an arithmetic sequence and the n th term is found using
=
+ (n - 1)d
where
is the first term and n = 21
thus
= 84 + (20 × - 9) = 84 - 180 = - 96