Question

What is the general term equation, an, for the arithmetic sequence 9, 5, 1, −3, . . . , and what is the 21st term of this sequence?

Answer 1

Hello : 
the general term equation, an, for the arithmetic sequence is : 
An = A1+d(n-1)
A1 : the first term   and : d the common diference
in this exercice : 
A1 = 9
d =(-3)-(1) =(1)-(5) =(5) -(9) = -4
An = 9-4(n-1)
An = -4n +13