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
[tex]a_{n}[/tex] = [tex]a_{1}[/tex] + (n - 1)d
where [tex]a_{1}[/tex] is the first term and n = 21
thus [tex]a_{21}[/tex] = 84 + (20 × - 9) = 84 - 180 = - 96
