400. Calculate the force of water pressure on a dam that has the shape of a trapezoid, where the upper base, coinciding with the water surface, is $10 \mathrm{~m}$ long, the lower base is $20 \mathrm{~m}$, and the height is $3 \mathrm{m}$.
Solution. Using formula (6), we find

$$
P=9810 \frac{(10+2 \cdot 20) 3^{2}}{6}=9810 \cdot \frac{450}{6}=735750(\mathrm{H})
$$