The plane is infinite and can be extended infinitely, but the plane figure does have size. So it is wrong for a plane figure to be a plane. We usually say that a plane can be represented by a plane figure in a plane, but it does not mean that the figure is a plane.
Parallelogram definition
Two sets of parallelograms whose opposite sides are parallel are called parallelograms.
1. A parallelogram is a planar figure.
2. A parallelogram is a quadrilateral.
3. A parallelogram is a centrosymmetric figure.
Parallelogram properties
(1) If a quadrilateral is a parallelogram, the two opposite sides of the quadrilateral are equal.
(Briefly described as "two sets of opposite sides of parallelogram are equal respectively")
(2) If a quadrilateral is a parallelogram, then the two sets of diagonals of the quadrilateral are equal.
(Briefly described as "two sets of diagonals of parallelogram are equal respectively")
(3) If a quadrilateral is a parallelogram, the neighboring angles of the quadrilateral are complementary.
(Briefly described as "the complementation of neighboring angles of parallelogram")
(4) The parallel heights sandwiched between two parallel lines are equal. (Briefly described as "the high distance between parallel lines is equal everywhere")
(5) If a quadrilateral is a parallelogram, the two diagonals of the quadrilateral are equally divided with each other.
(Briefly described as "the diagonals of parallelogram are equally divided with each other")
(6) The graph obtained by connecting the midpoints of the sides of an arbitrary quadrilateral is a parallelogram. (corollary)
(7) The area of a parallelogram is equal to the product of the base and the height. (It can be regarded as a rectangle.)
(8) A straight line passing through the intersection of the diagonals of a parallelogram to divide the parallelogram into two congruent parts