Step 1: Use the formula for area of a triangle.
If two sides of a triangle are \(a\) and \(b\), and the angle between them is \(\theta\), then the area is
\[
A=\frac{1}{2}ab\sin\theta
\]
Step 2: Identify the variable quantity.
Here, the two sides \(a\) and \(b\) are fixed.
So,
\[
\frac{1}{2}ab
\]
is constant.
Therefore, the area depends only on
\[
\sin\theta
\]
Step 3: Find when the area is maximum.
The maximum value of
\[
\sin\theta
\]
is
\[
1
\]
This happens when
\[
\theta=90^\circ
\]
Step 4: Final conclusion.
Hence, the angle between the given sides is
\[
\boxed{90^\circ}
\]