First, the rectangle
The essence of a rectangle is: two diagonal lines are equal; Two diagonal lines are equally divided; Two groups of opposite sides are parallel respectively; The two groups of opposite sides are equal respectively; All four corners are right angles; There are two symmetrical axes (there are four squares); Unstable (easily deformed); The square of the diagonal length of a rectangle is the sum of the squares of both sides; The quadrilateral obtained by connecting the midpoints of the sides of the rectangle in turn is a diamond.
Second, trapezoid.
A trapezoid is a quadrilateral with only one set of parallel opposite sides. The two parallel sides are called the bottom of the trapezoid, the longer bottom is called the lower bottom, and the shorter bottom is called the upper bottom. The other two sides are called waist; The vertical section sandwiched between the two base sides is called the height of the trapezoid. A trapezoid whose waist is perpendicular to the bottom is called a right-angled trapezoid. An isosceles trapezoid is called an isosceles trapezoid. Isosceles trapezoid is a special trapezoid, and its judgment method is similar to isosceles triangle.
Three, parallelogram
Parallelogram is a closed figure composed of two groups of parallel lines on the same two-dimensional plane. Parallelogram is generally named by the graphic name followed by four vertices. In Euclidean geometry, a parallelogram is a simple (non-self-intersecting) quadrilateral with two pairs of parallel sides. The opposite sides of a parallelogram are equal in length, and the diagonals of the parallelogram are equal.
Fourth, triangle.
Triangle is a closed figure composed of three line segments on the same plane but not on the same straight line, which has applications in mathematics and architecture. Ordinary triangles are divided into ordinary triangles (three sides are unequal) and isosceles triangles (isosceles triangles with unequal waist and bottom and isosceles triangles with equal waist and bottom, that is, equilateral triangles); According to the angle, there are right triangle, acute triangle and obtuse triangle, among which acute triangle and obtuse triangle are collectively called oblique triangle.
Five, other polygons
Polygon, in mathematical terms, a plane figure composed of three or more line segments connected end to end is called polygon. According to different standards, polygons can be divided into regular polygons and irregular polygons, convex polygons and concave polygons.