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\[ x + \frac{x^2}{2} + \frac{x^3}{3} - \log\left| 1 - x \right| + C\]
\[ x + \frac{x^2}{2} - \frac{x^3}{3} - \log\left| 1 - x \right| + C\]
\[ x - \frac{x^2}{2} - \frac{x^3}{3} - \log\left| 1 + x \right| + C\]
- \[ x - \frac{x^2}{2} + \frac{x^3}{3} - \log\left| 1 + x \right| + C\]
Solution
\[ x - \frac{x^2}{2} + \frac{x^3}{3} - \log\left| 1 + x \right| + C\]
\[\text{Let }I = \int\frac{x^3}{x + 1}dx\]
\[ = \int\frac{x^3 + 1 - 1}{x + 1}dx\]
\[ = \int\left( \frac{x^3 + 1}{x + 1} - \frac{1}{x + 1} \right)dx\]
\[ = \int\left( \frac{\left( x + 1 \right)\left( x^2 - x + 1 \right)}{x + 1} - \frac{1}{x + 1} \right)dx\]
\[ = \int\left( x^2 - x + 1 - \frac{1}{x + 1} \right)dx\]
\[ = \left( \frac{x^3}{3} - \frac{x^2}{2} + x - \log\left| x + 1 \right| \right) + C\]
\[ = \frac{x^3}{3} - \frac{x^2}{2} + x - \log\left| x + 1 \right| + C\]
\[\text{Therefore, }\int\frac{x^3}{x + 1}dx = \frac{x^3}{3} - \frac{x^2}{2} + x - \log\left| x + 1 \right| + C\]
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