Question: I = \int tg^{2}(x) \, dx
1. **State the problem:** We need to find the integral $$I = \int \tan^{2}(x) \, dx$$.
2. **Recall the identity:** Use the trigonometric identity $$\tan^{2}(x) = \sec^{2}(x) - 1$$.
3. **Rewrite the integral:** Substitute the identity into the integral:
$$I = \int (\sec^{2}(x) - 1) \, dx = \int \sec^{2}(x) \, dx - \int 1 \, dx$$.
4. **Integrate each term:**
- The integral of $$\sec^{2}(x)$$ is $$\tan(x) + C$$.
- The integral of $$1$$ is $$x + C$$.
5. **Combine results:**
$$I = \tan(x) - x + C$$.
**Final answer:**
$$\boxed{\int \tan^{2}(x) \, dx = \tan(x) - x + C}$$