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The sequence given is an Arithmetic Progression.

Arithmetic Sequences are characterized by:
The first term=8
The last term=12
Common Difference=-5

You can opt to solve for the nth term of the progression, using the formula:
An = A1 + (n-1) d

where An=the value of the nth term
A1= first term (which is 8)
d= common difference

For example, we are looking for the 8th term. So:
8th term = 8 + (8-1) x -5
8th term = -27

We can also solve for the sum of the Arithmetic Series, using the formula:
Sn = n/2 x (A1 + An)