1. To fold at a 90-degree angle, you can follow this step: fold the paper in half horizontally so that the left and right parts overlap each other. At this point, the crease line divides the paper into two isosceles right triangles.

Fold a right angle of the paper upward so that the right angle coincides with the horizontal crease line. At this point, this right angle becomes a new crease line, which forms a 90-degree angle with another right angle.

2. To fold it into a 45-degree angle, you can follow this step: fold the paper in half according to the length, so that the upper and lower parts overlap each other. At this point, the crease line divides the paper into two isosceles right triangles. Fold a right angle of the paper upward so that the right angle coincides with the horizontal crease line. At this point, this right angle becomes a new crease line, making an angle of 45 degrees with another right angle.

3. To fold an angle of 135 degrees, you can follow the following steps: fold the paper in half according to the length, so that the upper and lower parts overlap each other. At this point, the crease line divides the paper into two isosceles right triangles. Fold a right angle of the paper down so that it coincides with the horizontal crease line. At this point, this right angle becomes a new polyline and forms an angle of 135 degrees with another right angle.

Representation of mathematical degrees:

1, degree: This is the most commonly used method to express angles. The angle system stipulates that a complete circle is 360 degrees. Once round 1/360, it can be expressed as a fraction. For example, 45 degrees can be expressed as 45/360, that is, the numerator is 45 and the denominator is 360.

2. Curvature system: Curvature system is a commonly used method to express angles in the world. A complete circle is 2π radians. The relationship between radian and degree is: 1 radian equals 180/π degree. For example, 45 degrees can be expressed as 45/ 180π, that is, the numerator is 45 and the denominator is 180π.

3. Gradian: Gradian is an angle expression used by Germany, the Soviet Union and other countries. A complete circle is 400 degrees. Used to be equal to 1/400 perimeter. For example, 45 degrees can be expressed as 45/400, that is, the numerator is 45 and the denominator is 400.