Let $ABCD$ be a cyclic quadrilateral such that $AB \cdot BC=2 \cdot AD \cdot DC$. Prove that its diagonals $AC$ and $BD$ satisfy the inequality $8BD^2 \le 9AC^2$.
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1. **Given Condition and Setup:**
   We are given a cyclic quadrilateral \(ABCD\) such that \(AB \cdot BC = 2 \cdot AD \cdot DC\). We need to prove that the diagonals \(AC\) and \(BD\) satisfy the inequality \(8BD^2 \le 9AC^2\).

2. **Area Relationship:**
   The given condition can be interpreted in terms of areas. Specifically, it implies that the area of triangle \(ABC\) is twice the area of triangle \(ACD\). This is because the product of the sides around the common vertex \(C\) is proportional to the areas of the triangles sharing that vertex.

3. **Intersection of Diagonals:**
   Let \(E\) be the point of intersection of diagonals \(AC\) and \(BD\). We need to analyze the triangles formed by this intersection.

4. **Similarity of Triangles:**
   From the given condition, we can infer that \(\Delta DHE \sim \Delta BHE\), where \(H_1\) and \(H_2\) are the feet of the perpendiculars from \(D\) and \(B\) to \(AC\), respectively. This similarity gives us the ratio of the segments:
   \[
   \frac{BE}{ED} = 2
   \]
   Therefore, \(BE = 2ED\).

5. **Using the Ratio:**
   Since \(BE = 2ED\), let \(ED = x\) and \(BE = 2x\). The length of \(BD\) is then:
   \[
   BD = BE + ED = 2x + x = 3x
   \]

6. **Length of \(AC\):**
   The length of \(AC\) can be expressed in terms of \(AE\) and \(EC\). Using the fact that \(E\) divides \(AC\) into two segments, we have:
   \[
   AC = AE + EC
   \]
   By the properties of similar triangles and the given ratio, we can write:
   \[
   AC^2 = (AE + EC)^2 \ge 4 \cdot AE \cdot EC
   \]
   Since \(AE \cdot EC\) is proportional to \(BE \cdot ED\), we have:
   \[
   AE \cdot EC = BE \cdot ED = 2x \cdot x = 2x^2
   \]

7. **Inequality Derivation:**
   Substituting back, we get:
   \[
   AC^2 \ge 4 \cdot 2x^2 = 8x^2
   \]
   Since \(BD = 3x\), we have:
   \[
   BD^2 = (3x)^2 = 9x^2
   \]
   Therefore:
   \[
   9AC^2 \ge 8BD^2
   \]

8. **Conclusion:**
   We have shown that:
   \[
   9AC^2 \ge 8BD^2
   \]
   which implies:
   \[
   8BD^2 \le 9AC^2
   \]
   This completes the proof.

\(\blacksquare\)

The final answer is \( \boxed{ 8BD^2 \le 9AC^2 } \)