We are given that \( a, b, c \) are in arithmetic progression (A.P.).
In an A.P., the common difference is constant, i.e., \( b - a = c - b \). Therefore:
\[
b - a = c - b \quad \Rightarrow \quad 2b = a + c.
\]
Now, we need to compute \( \frac{a - b}{b - c} \). Using the relationship \( b - a = c - b \), we can write:
\[
\frac{a - b}{b - c} = \frac{-(b - a)}{b - c} = \frac{-(b - c)}{b - c} = -1.
\]
Thus, the value of \( \frac{a - b}{b - c} \) is \( 1 \).