1. From a square steel sheet with a side of 1 meter, identical rectangular isosceles triangles are cut from each of the four corners, leaving a smaller square. Determine its mass if the thickness of the sheet is 3 mm and the density of steel is 7.8 $g / \mathrm{cm}^{3}$.
Answer: 11.7 kg.

Solution. The area of the resulting square is half the area of the original square and is $\frac{10000}{2}=5000\left(\mathrm{~cm}^{2}\right)$, its volume is $5000 \cdot 0.3=1500\left(\mathrm{~cm}^{3}\right)$, and its mass is $1500 \cdot 7.8=11700(g)$.