What is the largest possible area of a quadrilateral with sidelengths $1, 4, 7$ and $8$ ?
1. **Identify the problem and given data:**
   We need to find the largest possible area of a quadrilateral with side lengths \(1, 4, 7,\) and \(8\).

2. **Use Bretschneider's formula:**
   The area \(A\) of a general quadrilateral with sides \(a, b, c, d\) and angles \(\alpha\) and \(\gamma\) opposite each other is given by:
   \[
   A = \sqrt{(s-a)(s-b)(s-c)(s-d) - abcd \cos^2\left(\frac{\alpha + \gamma}{2}\right)}
   \]
   where \(s\) is the semiperimeter:
   \[
   s = \frac{a + b + c + d}{2}
   \]

3. **Maximize the area:**
   The area is maximized when the quadrilateral is cyclic (inscribed in a circle), which simplifies Bretschneider's formula to Brahmagupta's formula:
   \[
   A = \sqrt{(s-a)(s-b)(s-c)(s-d)}
   \]
   This happens when \(\alpha + \gamma = 180^\circ\), making \(\cos\left(\frac{\alpha + \gamma}{2}\right) = 0\).

4. **Check if the quadrilateral can be cyclic:**
   For a quadrilateral to be cyclic, the sum of the opposite angles must be \(180^\circ\). We can use the Ptolemy's theorem for cyclic quadrilaterals:
   \[
   ac + bd = pq
   \]
   where \(a, b, c, d\) are the sides and \(p, q\) are the diagonals.

5. **Verify the cyclic condition:**
   We need to check if the given sides satisfy the condition for a cyclic quadrilateral. We use the fact that the sum of the squares of the opposite sides should be equal:
   \[
   1^2 + 8^2 = 4^2 + 7^2
   \]
   \[
   1 + 64 = 16 + 49
   \]
   \[
   65 = 65
   \]
   This confirms that the quadrilateral can be inscribed in a circle.

6. **Calculate the semiperimeter:**
   \[
   s = \frac{1 + 4 + 7 + 8}{2} = 10
   \]

7. **Apply Brahmagupta's formula:**
   \[
   A = \sqrt{(10-1)(10-4)(10-7)(10-8)}
   \]
   \[
   A = \sqrt{9 \cdot 6 \cdot 3 \cdot 2}
   \]
   \[
   A = \sqrt{324} = 18
   \]

8. **Conclusion:**
   The largest possible area of the quadrilateral is \(18\).

The final answer is \(\boxed{18}\).