In the $x$-$y$ plane, for the origin $ O$, given an isosceles triangle $ OAB$ with $ AO \equal{} AB$ such that $ A$ is on the first quadrant and $ B$ is on the $ x$ axis. 
Denote the area by $ s$. Find the area of the common part of the traingle and the region expressed by the inequality $ xy\leq 1$ to give the area as the function of $ s$.
1. **Define the coordinates and area of the triangle:**
   Let \( B \) be at \( (2a, 0) \) and \( A \) be at \( (a, b) \) where \( a, b > 0 \). The area \( s \) of the triangle \( OAB \) is given by:
   \[
   s = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times 2a \times b = ab
   \]
   Therefore, \( s = ab \).

2. **Equation of the lines \( AB \) and \( OA \):**
   - The equation of line \( AB \) can be derived using the points \( A(a, b) \) and \( B(2a, 0) \):
     \[
     \text{Slope of } AB = \frac{0 - b}{2a - a} = -\frac{b}{a}
     \]
     Using point-slope form, the equation of \( AB \) is:
     \[
     y - b = -\frac{b}{a}(x - a) \implies bx + ay - 2ab = 0
     \]
   - The equation of line \( OA \) can be derived using the points \( O(0, 0) \) and \( A(a, b) \):
     \[
     \text{Slope of } OA = \frac{b - 0}{a - 0} = \frac{b}{a}
     \]
     Using point-slope form, the equation of \( OA \) is:
     \[
     y = \frac{b}{a}x \implies bx - ay = 0
     \]

3. **Case 1: \( s \in (0, 1] \)**
   - The line \( AB \) does not intersect the hyperbola \( xy = 1 \) or is tangent to it. This implies the quadratic equation \( bx^2 - 2abx + a = 0 \) has non-real or equal roots:
     \[
     \Delta = (2ab)^2 - 4 \cdot b \cdot a = 4a^2b^2 - 4ab = 4ab(ab - 1) \leq 0
     \]
     \[
     s(s - 1) \leq 0 \implies s \in (0, 1]
     \]
   - In this case, the entire triangle \( OAB \) is outside the hyperbola \( xy = 1 \). Therefore, the area of the common part is:
     \[
     \boxed{s}
     \]

4. **Case 2: \( s > 1 \)**
   - The line segment \( AB \) intersects the hyperbola \( xy = 1 \). This implies the quadratic equation \( bx^2 - 2abx + a = 0 \) has real and distinct roots:
     \[
     \Delta = 4ab(ab - 1) > 0 \implies s(s - 1) > 0 \implies s \in (1, \infty)
     \]
   - Let \( Q \) be the point of intersection of \( AB \) and the hyperbola. The \( x \)-coordinate of \( Q \) is:
     \[
     x_Q = \frac{s + \sqrt{s^2 - s}}{b}
     \]
   - Let \( P \) be the point of intersection of \( OA \) and the hyperbola. The \( x \)-coordinate of \( P \) is:
     \[
     x_P = \frac{\sqrt{s}}{b}
     \]
   - The required area is the area of the triangle minus the area under the curve \( xy = 1 \) between \( P \) and \( Q \):
     \[
     \text{Required area} = s - \left( \int_{\frac{\sqrt{s}}{b}}^a \left( \frac{bx}{a} - \frac{1}{x} \right) dx + \int_{a}^{\frac{s + \sqrt{s^2 - s}}{b}} \left( 2b - \frac{bx}{a} - \frac{1}{x} \right) dx \right)
     \]
     \[
     = s - \sqrt{s^2 - s} - \ln(\sqrt{s} - \sqrt{s - 1})
     \]