Cyclic quadrilateral $ABCD$ has $AC\perp BD$, $AB+CD=12$, and $BC+AD=13$. FInd the greatest possible area of $ABCD$. 
1. **Identify the given conditions and variables:**
   - Cyclic quadrilateral \(ABCD\) with \(AC \perp BD\).
   - \(AB + CD = 12\).
   - \(BC + AD = 13\).
   - Let the intersection of diagonals \(AC\) and \(BD\) be \(P\).
   - Let \(AB = a\), \(BC = b\), \(CD = c\), \(AD = d\), \(AP = w\), \(BP = z\), \(CP = x\), \(DP = y\).

2. **Apply Ptolemy's Theorem:**
   - For a cyclic quadrilateral, Ptolemy's Theorem states:
     \[
     AB \cdot CD + AD \cdot BC = AC \cdot BD
     \]
   - Since \(AC \perp BD\), the area of \(ABCD\) can be expressed as:
     \[
     [ABCD] = \frac{1}{2} \times AC \times BD
     \]

3. **Express the area in terms of the sides:**
   - Using the given conditions \(AB + CD = 12\) and \(BC + AD = 13\), we need to maximize the area.
   - By the Pythagorean theorem in the right triangles formed by the diagonals:
     \[
     a^2 = w^2 + z^2, \quad b^2 = z^2 + x^2, \quad c^2 = x^2 + y^2, \quad d^2 = y^2 + w^2
     \]
   - Adding and subtracting these equations:
     \[
     a^2 + c^2 = w^2 + z^2 + x^2 + y^2 + x^2 + y^2 = b^2 + d^2
     \]
     \[
     a^2 + c^2 = b^2 + d^2
     \]

4. **Maximize the product \(ac\) and \(bd\):**
   - Given \(a + c = 12\) and \(b + d = 13\), we use the AM-GM inequality:
     \[
     \frac{a + c}{2} \geq \sqrt{ac} \implies \frac{12}{2} \geq \sqrt{ac} \implies 6 \geq \sqrt{ac} \implies 36 \geq ac
     \]
     \[
     \frac{b + d}{2} \geq \sqrt{bd} \implies \frac{13}{2} \geq \sqrt{bd} \implies \frac{169}{4} \geq bd
     \]

5. **Solve for \(ac\) and \(bd\):**
   - To maximize the area, we need to maximize \(ac + bd\):
     \[
     2bd = 25 + 2ac \implies bd = \frac{25}{2} + ac
     \]
     \[
     \frac{169}{4} = \frac{25}{2} + ac \implies ac = \frac{119}{4}
     \]

6. **Calculate the area:**
   - The area of \(ABCD\) is given by:
     \[
     [ABCD] = \frac{1}{2} \times (ac + bd) = \frac{1}{2} \times \left(\frac{119}{4} + \frac{169}{4}\right) = \frac{1}{2} \times \frac{288}{4} = \frac{1}{2} \times 72 = 36
     \]

The final answer is \(\boxed{36}\).