(1) Evaluate $ \int_1^{3\sqrt{3}} \left(\frac{1}{\sqrt[3]{x^2}}\minus{}\frac{1}{1\plus{}\sqrt[3]{x^2}}\right)\ dx.$

(2) Find the positive real numbers $ a,\ b$ such that for $ t>1,$ $ \lim_{t\rightarrow \infty} \left(\int_1^t \frac{1}{1\plus{}\sqrt[3]{x^2}}\ dx\minus{}at^b\right)$ converges.
### Part 1: Evaluate the integral

1. **Substitution**: Let \( \sqrt[3]{x^2} = \tan^2 u \). Then, \( x = (\tan^2 u)^{3/2} = \tan^3 u \).

2. **Differentiation**: Differentiate both sides with respect to \( u \):
   \[
   dx = 3 \tan^2 u \sec^2 u \, du
   \]

3. **Change of limits**: When \( x = 1 \), \( \sqrt[3]{1^2} = 1 \Rightarrow \tan^2 u = 1 \Rightarrow u = \frac{\pi}{4} \). When \( x = 3\sqrt{3} \), \( \sqrt[3]{(3\sqrt{3})^2} = 3 \Rightarrow \tan^2 u = 3 \Rightarrow u = \frac{\pi}{3} \).

4. **Rewrite the integral**:
   \[
   \int_1^{3\sqrt{3}} \left( \frac{1}{\sqrt[3]{x^2}} - \frac{1}{1 + \sqrt[3]{x^2}} \right) dx = \int_{\frac{\pi}{4}}^{\frac{\pi}{3}} \left( \frac{1}{\tan^2 u} - \frac{1}{1 + \tan^2 u} \right) 3 \tan^2 u \sec^2 u \, du
   \]

5. **Simplify the integrand**:
   \[
   \frac{1}{\tan^2 u} - \frac{1}{1 + \tan^2 u} = \cot^2 u - \frac{1}{\sec^2 u} = \cot^2 u - \cos^2 u
   \]
   Since \( \cot^2 u = \frac{\cos^2 u}{\sin^2 u} \) and \( \sec^2 u = 1 + \tan^2 u \), the integrand simplifies to:
   \[
   \cot^2 u - \cos^2 u = \frac{\cos^2 u}{\sin^2 u} - \cos^2 u = \frac{\cos^2 u - \cos^4 u}{\sin^2 u} = \frac{\cos^2 u (1 - \cos^2 u)}{\sin^2 u} = \frac{\cos^2 u \sin^2 u}{\sin^2 u} = \cos^2 u
   \]

6. **Integrate**:
   \[
   \int_{\frac{\pi}{4}}^{\frac{\pi}{3}} 3 \cos^2 u \, du = 3 \int_{\frac{\pi}{4}}^{\frac{\pi}{3}} \cos^2 u \, du
   \]
   Using the identity \( \cos^2 u = \frac{1 + \cos 2u}{2} \):
   \[
   3 \int_{\frac{\pi}{4}}^{\frac{\pi}{3}} \frac{1 + \cos 2u}{2} \, du = \frac{3}{2} \int_{\frac{\pi}{4}}^{\frac{\pi}{3}} (1 + \cos 2u) \, du
   \]
   \[
   = \frac{3}{2} \left[ u + \frac{\sin 2u}{2} \right]_{\frac{\pi}{4}}^{\frac{\pi}{3}}
   \]
   \[
   = \frac{3}{2} \left( \left( \frac{\pi}{3} + \frac{\sin \frac{2\pi}{3}}{2} \right) - \left( \frac{\pi}{4} + \frac{\sin \frac{\pi}{2}}{2} \right) \right)
   \]
   \[
   = \frac{3}{2} \left( \frac{\pi}{3} - \frac{\pi}{4} + \frac{\sqrt{3}}{4} - \frac{1}{2} \right)
   \]
   \[
   = \frac{3}{2} \left( \frac{4\pi - 3\pi}{12} + \frac{\sqrt{3} - 2}{4} \right)
   \]
   \[
   = \frac{3}{2} \left( \frac{\pi}{12} + \frac{\sqrt{3} - 2}{4} \right)
   \]
   \[
   = \frac{3}{2} \left( \frac{\pi + 3(\sqrt{3} - 2)}{12} \right)
   \]
   \[
   = \frac{3}{2} \cdot \frac{\pi + 3\sqrt{3} - 6}{12}
   \]
   \[
   = \frac{3(\pi + 3\sqrt{3} - 6)}{24}
   \]
   \[
   = \frac{\pi + 3\sqrt{3} - 6}{8}
   \]

### Part 2: Find \(a\) and \(b\)

1. **Integral**: Consider the integral:
   \[
   \int_1^t \frac{1}{1 + \sqrt[3]{x^2}} \, dx
   \]

2. **Substitution**: Let \( \sqrt[3]{x^2} = \tan^2 u \). Then, \( x = \tan^3 u \) and \( dx = 3 \tan^2 u \sec^2 u \, du \).

3. **Change of limits**: When \( x = 1 \), \( u = \frac{\pi}{4} \). When \( x = t \), \( u = \tan^{-1} \sqrt[3]{t} \).

4. **Rewrite the integral**:
   \[
   \int_1^t \frac{1}{1 + \sqrt[3]{x^2}} \, dx = \int_{\frac{\pi}{4}}^{\tan^{-1} \sqrt[3]{t}} \frac{1}{1 + \tan^2 u} 3 \tan^2 u \sec^2 u \, du
   \]

5. **Simplify the integrand**:
   \[
   \frac{1}{1 + \tan^2 u} = \cos^2 u
   \]
   \[
   \int_{\frac{\pi}{4}}^{\tan^{-1} \sqrt[3]{t}} 3 \tan^2 u \cos^2 u \sec^2 u \, du = 3 \int_{\frac{\pi}{4}}^{\tan^{-1} \sqrt[3]{t}} \tan^2 u \, du
   \]

6. **Integrate**:
   \[
   3 \int_{\frac{\pi}{4}}^{\tan^{-1} \sqrt[3]{t}} \tan^2 u \, du = 3 \left[ \tan u - u \right]_{\frac{\pi}{4}}^{\tan^{-1} \sqrt[3]{t}}
   \]
   \[
   = 3 \left( \tan (\tan^{-1} \sqrt[3]{t}) - \tan^{-1} \sqrt[3]{t} - \left( \tan \frac{\pi}{4} - \frac{\pi}{4} \right) \right)
   \]
   \[
   = 3 \left( \sqrt[3]{t} - \tan^{-1} \sqrt[3]{t} - (1 - \frac{\pi}{4}) \right)
   \]

7. **Limit**: For the limit to exist, we need:
   \[
   \lim_{t \to \infty} \left( \int_1^t \frac{1}{1 + \sqrt[3]{x^2}} \, dx - at^b \right) = \lim_{t \to \infty} \left( 3 \sqrt[3]{t} - 3 \tan^{-1} \sqrt[3]{t} + \frac{3\pi}{4} - 3 - at^b \right)
   \]
   \[
   = \lim_{t \to \infty} \left( 3 \sqrt[3]{t} - at^b \right)
   \]
   For the limit to converge, the terms involving \( t \) must cancel out, so:
   \[
   3 \sqrt[3]{t} = at^b
   \]
   \[
   b = \frac{1}{3}, \quad a = 3
   \]

The final answer is \( \boxed{ a = 3, b = \frac{1}{3} } \)