On the coordinate plane, find the area of the part enclosed by the curve $C: (a+x)y^2=(a-x)x^2\ (x\geq 0)$ for $a>0$.
1. **Identify the curve and its symmetry:**
   The given curve is \( (a + x)y^2 = (a - x)x^2 \) for \( x \geq 0 \) and \( a > 0 \). The curve is symmetric about the \( x \)-axis and intersects the \( x \)-axis at \( x = 0 \) and \( x = a \).

2. **Set up the integral for the area:**
   Since the curve is symmetric about the \( x \)-axis, we can find the area of the upper half and then double it to get the total area. The area \( S \) of the loop is given by:
   \[
   S = 2 \int_0^a y \, dx
   \]
   From the given equation, solve for \( y \):
   \[
   y^2 = \frac{(a - x)x^2}{a + x}
   \]
   Therefore,
   \[
   y = \sqrt{\frac{(a - x)x^2}{a + x}}
   \]

3. **Simplify the integral:**
   Substitute \( y \) into the integral:
   \[
   S = 2 \int_0^a \sqrt{\frac{(a - x)x^2}{a + x}} \, dx
   \]
   Simplify the integrand:
   \[
   S = 2 \int_0^a \frac{x \sqrt{a - x}}{\sqrt{a + x}} \, dx
   \]

4. **Break the integral into parts:**
   Let \( I = \int_0^a \frac{x \sqrt{a - x}}{\sqrt{a + x}} \, dx \). We can use the substitution \( x = a \sin^2 \theta \), which implies \( dx = 2a \sin \theta \cos \theta \, d\theta \). The limits change from \( x = 0 \) to \( x = a \) as \( \theta \) goes from \( 0 \) to \( \frac{\pi}{2} \).

5. **Perform the substitution:**
   \[
   I = \int_0^{\frac{\pi}{2}} \frac{a \sin^2 \theta \sqrt{a - a \sin^2 \theta}}{\sqrt{a + a \sin^2 \theta}} \cdot 2a \sin \theta \cos \theta \, d\theta
   \]
   Simplify the integrand:
   \[
   I = 2a^2 \int_0^{\frac{\pi}{2}} \frac{\sin^3 \theta \cos \theta \sqrt{1 - \sin^2 \theta}}{\sqrt{1 + \sin^2 \theta}} \, d\theta
   \]
   Since \( \sqrt{1 - \sin^2 \theta} = \cos \theta \):
   \[
   I = 2a^2 \int_0^{\frac{\pi}{2}} \frac{\sin^3 \theta \cos^2 \theta}{\sqrt{1 + \sin^2 \theta}} \, d\theta
   \]

6. **Evaluate the integral:**
   This integral can be evaluated using standard techniques or tables of integrals. However, for simplicity, we can use the given solution's approach to break it into parts:
   \[
   S = 2 \left( I_1 + I_2 - I_3 \right)
   \]
   where:
   \[
   I_1 = \int_0^a \frac{ax}{\sqrt{a^2 - x^2}} \, dx = \left. -a \sqrt{a^2 - x^2} \right|_0^a = a^2
   \]
   \[
   I_2 = \int_0^a \sqrt{a^2 - x^2} \, dx = \left. \frac{x \sqrt{a^2 - x^2}}{2} + \frac{a^2}{2} \arcsin \frac{x}{a} \right|_0^a = \frac{\pi a^2}{4}
   \]
   \[
   I_3 = \int_0^a \frac{a^2}{\sqrt{a^2 - x^2}} \, dx = \left. a^2 \arcsin \frac{x}{a} \right|_0^a = \frac{\pi a^2}{2}
   \]

7. **Combine the results:**
   \[
   S = 2 \left( a^2 + \frac{\pi a^2}{4} - \frac{\pi a^2}{2} \right) = 2 \left( a^2 + \frac{\pi a^2}{4} - \frac{2\pi a^2}{4} \right) = 2 \left( a^2 - \frac{\pi a^2}{4} \right)
   \]
   \[
   S = 2a^2 - \frac{\pi a^2}{2} = \frac{a^2}{2} (4 - \pi)
   \]

The final answer is \(\boxed{\frac{a^2}{2} (4 - \pi)}\).