5. (15 points) As shown in the figure, the side length of the square is 80 cm, $O$ is the center of the square, $A$ is the midpoint of $O B$, and within the square, a circle is drawn with $A$ as the center and $O A$ as the radius. Another circle is drawn with $B$ as the center and $O B$ as the radius, forming a special shape with one side of the square. Rotating this shape around point $O$ three times in a clockwise direction forms a windmill shape. What is the area of this windmill (shaded part) in square centimeters? (Take $\pi$ as 3.14)


【Solution】Solution: According to the problem, we have:
The area of triangle $OBC$ in the figure is $80 \times 80 \div 4 = 1600$ (square centimeters). Therefore, $\frac{1}{2} OB^2 = 1600. OB^2 = 3200$.
$\because \angle OBC = 45^{\circ}$. One-eighth of the circle's area is $\frac{1}{8} \pi OB^2 = 400 \times 3.14 = 1256$ (square centimeters). $OA^2 = \left(\frac{1}{2} OB\right)^2 = 800$. One-quarter of the circle's area is: $\frac{1}{4} \pi OA^2 = 628$ (square centimeters). The area of the small triangle is one-fourth of the entire triangle $OBC$. $1600 \div 4 = 400$ (square centimeters).

The area of a small shaded region is: $1256 - 628 - 400 = 228$ (square centimeters).
The total shaded area is: $228 \times 4 = 912$ (square centimeters).
Therefore, the answer is: 912