The cover of mathematics textbook is rectangular, which is one of the common figures in plane geometry. It has four corners. Every angle is a right angle, which is 90 degrees. In a rectangle, opposite sides are equal and parallel, and equal angles are right angles. So the cover of a math textbook can be regarded as a rectangle with four corners, each of which is a right angle.

When you open the math textbook, you can clearly see the four corners of the cover, which are the intersection of the corner of the page and the line of the cover. These corners not only constitute the basic shape of the cover, but also are an important part of the cover design.

Characteristics of angle:

1, measurement of angle: the size of an angle can be measured by measurement, usually in degrees. In plane geometry, the angle is fixed and can be measured with a protractor. For angles less than 180 degrees, we can get the degree by direct measurement; For an angle greater than or equal to 180 degrees, we can get its degree by measuring its complementary angle.

2. Types of angles: According to the relative positions of two rays, angles can be divided into acute angle, right angle, obtuse angle and equal angle. Acute angle refers to an angle less than 90 degrees, right angle refers to an angle equal to 90 degrees, obtuse angle refers to an angle greater than 90 degrees but less than 180 degrees, and equiangular angle refers to an angle equal to 180 degrees. In addition, the angle can also be divided into convex angle and parallel angle. A convex angle refers to the angle at which two rays intersect outside, and a parallel angle refers to the angle at which two rays intersect on the same straight line.

3. bisector of an angle: an angle can be divided into two equal parts, and the intersection of the bisector is called the vertex of the angle. The bisector of an angle is one of the most important concepts in geometry. It can divide an angle into two equal parts, and the degree of each part is half of the original angle.

4. Equality and complementarity of angles: if the degrees of two angles are equal, they are equal angles; If the sum of the degrees of two angles is equal to 180 degrees, then they are complementary angles. Equality and complementarity are two very important concepts in geometry, which have applications in many mathematical problems.

5. Application of angles: angles are used in many mathematical problems, such as the theorem of the sum of internal angles of triangles (the sum of three internal angles is equal to 180 degrees) and the calculation of the sum of internal angles of polygons. Besides, in physics and engineering, the concept of angle is often used to solve various problems.