For example, lines, rays, angles, triangles, parallelograms, rectangles (squares), trapeziums, circles, etc. are all geometric figures, and the parts represented by these figures are all in the same plane, which is called plane figures. Points, lines and corners are all plane figures.

Geometric graphics, that is, all kinds of graphics abstracted from physical objects, can help people effectively describe the complex world. There are geometric figures everywhere in life, and everything we see is composed of basic geometric figures such as points, lines and surfaces. Geometry originated from western geodesy, which solves the relationship between points, lines and surfaces. The endless changes make the geometric pattern itself have endless charm.

Do triangles and trapeziums have to be plane figures? Two points define a straight line and three points define a plane, so a triangle must be a plane figure, but a trapezoid is not.

Why quadrilateral is not necessarily a plane figure?

Any three points of a quadrilateral must be on a plane, and three points determine a plane, so a triangle must be a plane figure, but whether a quadrilateral is a plane figure depends on the fourth point. If the fourth point is in the plane determined by the first three points, it is a plane quadrilateral, and if the fourth point is out of the plane, it is a space quadrilateral. It is uncertain whether a quadrilateral is a plane quadrilateral or a spatial quadrilateral, so the book says that a quadrilateral is not necessarily a plane figure.

Plane figure expansion angle

It is an important concept in mathematics, which is often involved in junior high school mathematics. It means that in a plane figure, there is a line segment as the baseline, and the inner or outer angles on both sides of it expand into a new angle. The angle that changes compared with the original angle is called the unfolding angle. The following is a detailed, professional and comprehensive summary of the knowledge points about the expansion angle of plane graphics.

Concept of expansion angle of plane figure

The expansion angle of a plane figure refers to the expansion angle formed by the expansion of the inner angle or outer angle on both sides of a line segment into a new angle in plane geometry.