A parallelogram is a figure with a symmetrical center, and the intersection of diagonal lines is its symmetrical center. The diagonals of parallelogram are equal.
Diagonal lines of parallelograms are equally divided. A set of parallelograms whose opposite sides are parallel and equal is a parallelogram.
A quadrilateral with two diagonal lines bisecting each other is a parallelogram, and two groups of quadrilaterals with equal opposite sides are parallelograms.
Rectangle: A parallelogram with a right angle is called a rectangle. The diagonals of a rectangle are equal, and all four corners are right angles.
A quadrilateral with three right angles is a rectangle, and a parallelogram with equal diagonal angles is a rectangle.
Diamond: A group of parallelograms with equal adjacent sides is called a diamond. All four sides of a diamond are equal.
Diagonal lines of the diamond are perpendicular to each other, and each diagonal line bisects a set of diagonal lines.
A quadrilateral with four equal sides is a diamond, and a parallelogram with diagonal lines perpendicular to each other is a diamond.
Square: A group of parallelograms with equal adjacent sides and a right angle is called a square.
A square has both rectangular and rhombic properties.
Triangle: The line segment connecting the midpoints of two sides of a triangle is called the midline of the triangle.
The center line of a triangle is parallel to the third side and equal to half of it.
Trapezoid: The line segment connecting the midpoints of the two waists of the trapezoid is called the midline of the trapezoid.
The center line of the trapezoid is parallel to the two bottom sides and equal to half of the sum of the two bottom sides.