336. Find the condition that links three numbers $a, b, c$, if they are members of the same arithmetic progression.
Instruction. Suppose that the given numbers are terms of the same arithmetic progression with the first term $a_{1}$ and common difference $d$. Then

$$
a=a_{1}+d n, \quad b=a_{1}+d \kappa, \quad c=a_{1}+d p
$$

where $n, p, k$ - are integers. From these equations, we will have

$$
\frac{b-a}{c-b}=\frac{k-n}{p-k}=\frac{A}{B},
$$

where $A$ and $B$ - are integers.

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