242. Find $\int_{3}^{1} x^{2} d x$.
Solution. $\int_{3}^{1} x^{2} d x=-\int_{1}^{3} x^{2} d x=-\left.\frac{x^{3}}{3}\right|_{1} ^{3}=\left(-\frac{27}{3}\right)-\left(-\frac{1}{3}\right)=$ $=-\frac{26}{3}=-8 \frac{2}{3}$.