Find the positive value of $ a$ such that the curve $ C_1: x \equal{} \sqrt {2y^2 \plus{} \frac {25}{2}}$ tangent to the parabola $ C_2: y \equal{} ax^2$, then find the equation of the tangent line of $ C_1$ at the point of tangency.
1. **Rewrite the given curves:**
   - The curve \( C_1 \) is given by \( x = \sqrt{2y^2 + \frac{25}{2}} \). Squaring both sides, we get:
     \[
     x^2 = 2y^2 + \frac{25}{2}
     \]
   - The parabola \( C_2 \) is given by \( y = ax^2 \).

2. **Find the derivatives:**
   - Differentiate \( x^2 = 2y^2 + \frac{25}{2} \) implicitly with respect to \( x \):
     \[
     2x = 4y \frac{dy}{dx}
     \]
     Simplifying, we get:
     \[
     \frac{dy}{dx} = \frac{x}{2y}
     \]
   - Differentiate \( y = ax^2 \) with respect to \( x \):
     \[
     \frac{dy}{dx} = 2ax
     \]

3. **Set the derivatives equal at the point of tangency:**
   - At the point of tangency, the slopes of the tangents to both curves must be equal:
     \[
     \frac{x}{2y} = 2ax
     \]
     Solving for \( x \) and \( y \):
     \[
     \frac{1}{2y} = 2a \implies y = \frac{1}{4a}
     \]

4. **Substitute \( y = ax^2 \) into \( x^2 = 2y^2 + \frac{25}{2} \):**
   - Substitute \( y = ax^2 \) into \( x^2 = 2y^2 + \frac{25}{2} \):
     \[
     x^2 = 2(ax^2)^2 + \frac{25}{2}
     \]
     Simplifying, we get:
     \[
     x^2 = 2a^2x^4 + \frac{25}{2}
     \]
     Rearrange to form a quadratic in \( x^2 \):
     \[
     2a^2x^4 - x^2 + \frac{25}{2} = 0
     \]
     Let \( u = x^2 \):
     \[
     2a^2u^2 - u + \frac{25}{2} = 0
     \]

5. **Solve the quadratic equation for \( u \):**
   - Use the quadratic formula \( u = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \):
     \[
     u = \frac{1 \pm \sqrt{1 - 4 \cdot 2a^2 \cdot \frac{25}{2}}}{4a^2}
     \]
     Simplifying inside the square root:
     \[
     u = \frac{1 \pm \sqrt{1 - 50a^2}}{4a^2}
     \]
     For \( u \) to be real, the discriminant must be non-negative:
     \[
     1 - 50a^2 \geq 0 \implies a^2 \leq \frac{1}{50} \implies a \leq \frac{1}{\sqrt{50}} = \frac{1}{5\sqrt{2}}
     \]

6. **Find the value of \( a \):**
   - Given the point of tangency \( (5, \frac{5}{2}) \), substitute \( x = 5 \) and \( y = \frac{5}{2} \) into \( y = ax^2 \):
     \[
     \frac{5}{2} = a \cdot 25 \implies a = \frac{1}{10}
     \]

7. **Verify the point of tangency:**
   - Substitute \( a = \frac{1}{10} \) back into the equations:
     \[
     y = \frac{1}{10}x^2 \implies y = \frac{1}{10}(5^2) = \frac{25}{10} = \frac{5}{2}
     \]
     - Check \( x^2 = 2y^2 + \frac{25}{2} \):
     \[
     25 = 2 \left( \frac{5}{2} \right)^2 + \frac{25}{2} \implies 25 = 2 \cdot \frac{25}{4} + \frac{25}{2} \implies 25 = \frac{50}{4} + \frac{50}{4} = 25
     \]

8. **Find the equation of the tangent line:**
   - The slope of the tangent line at \( (5, \frac{5}{2}) \) is:
     \[
     \frac{dy}{dx} = 2a \cdot 5 = 2 \cdot \frac{1}{10} \cdot 5 = 1
     \]
   - Using the point-slope form of the line equation:
     \[
     y - \frac{5}{2} = 1(x - 5) \implies y = x - \frac{5}{2}
     \]
     Multiply through by 2 to clear the fraction:
     \[
     2y = 2x - 5 \implies 2x - 2y - 5 = 0
     \]

The final answer is \(\boxed{2x - 2y - 5 = 0}\)