Charice made a paper Christmas tree with three identical triangles and a square as shown. Find the area of the shaded part

To find the area of the shaded part of the Christmas tree consisting of three identical triangles and a square, we can break down the problem as follows:

1. **Square Area**:
The square at the bottom has a side length of \(10 \, \text{cm}\).
\[
\text{Area of square} = \text{side} \times \text{side} = 10 \, \text{cm} \times 10 \, \text{cm} = 100 \, \text{cm}^2.
\]

2. **Area of One Triangle**:
Each triangle has a base of \(28 \, \text{cm}\) and a height of \(20 \, \text{cm}\).
\[
\text{Area of one triangle} = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times 28 \, \text{cm} \times 20 \, \text{cm} = 280 \, \text{cm}^2.
\]

3. **Total Area of Triangles**:
Since there are three identical triangles, the total area of the triangles is:
\[
\text{Total area of triangles} = 3 \times 280 \, \text{cm}^2 = 840 \, \text{cm}^2.
\]

4. **Total Area of the Tree**:
The total area of the shaded part of the Christmas tree (which is the area of the triangles minus the area of the square) is calculated as follows:
\[
\text{Total area} = \text{Total area of triangles} - \text{Area of square} = 840 \, \text{cm}^2 - 100 \, \text{cm}^2 = 740 \, \text{cm}^2.
\]

Therefore, the area of the shaded part of the Christmas tree is **740 cm²**.