As shown in the figure, it is known that the area of the shaded part is 20cm^2 and the area of the ring-|||-shape is 62.8cm2. ..

Thinking guidance

It is known that the area of the shadow is 20 square centimeters, and the area of the shadow is equal to the area of the big square with the outer circle radius as the side length minus the area of the small square with the inner circle radius as the side length, and then it can be solved according to the annular area formula.

The method of finding the area of combined graphics;

1, cutting method: observe the graph, divide the graph into easy-to-find graphs, and then add and subtract.

2. Complement method: observe the figure, make up a part of the figure to form an easy-to-find figure, and then add and subtract;

3. Combination of cutting and mending: observe the figure, divide it, and then move and mend it to form an easy-to-find figure.

The area calculation method of combined graphics is to combine several regular basic graphics together to form a combined graphics. The general method to find the area of this combined figure is digging and filling method.

The way to learn math well is as follows:

First, master the basic knowledge, which is the basis of learning mathematics well.

Although mathematics is difficult for many students, for students who want to learn mathematics well, the first thing to do is to master the basic knowledge, which is the basis of learning mathematics well. The basic knowledge of mathematics is generally the knowledge of some concepts and formulas. Only by mastering these clearly can students have a good application and mathematics be improved.

Second, think more, thinking is the key to flexible use of knowledge.

Mathematics is a subject that attaches great importance to the logical thinking ability, so students should use knowledge flexibly in the process of learning, which is the key factor to learn mathematics well. Students' thinking is exercised in the process of continuous thinking. Only by thinking deeply often can their thinking be more active and their mathematics learning be easier.

Third, listen carefully in class.

It is also an important link to listen carefully in class if you want to learn math well. In class, the teacher usually talks about the idea of doing the problem and some expanded knowledge. If students can keep up with the teacher's thinking in class, then on the whole, such students have good math scores, so if they want to do well in math, they should listen carefully in class.