9th term = a+8d = 12
17th term= a+16d = 28
subtract them
8d = 16
d = 2
sub into a+8d = 12
a = -4
4th term = a+3d = -4 + 6 = 2
The 9th term of an arithmetic sequence is 12 and the 17th term is 28, find the 4th term of the arithmetic sequences
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