Parallelogram
In geometry, a parallelogram is a quadrilateral with two sets of parallel sides. The opposite sides of a parallelogram are of equal length, and the opposite angles of a parallelogram are congruent. The three-dimensional counterpart of a parallelogram is a parallelepiped.
Properties
- The two parallel sides are of equal length.
- The area, , of a parallelogram is where is the base of the parallelogram and is its height.
- The area of a parallelogram is twice the area of a triangle created by one of its diagonals.
- The area is also equal to the magnitude of the vector cross product of two adjacent sides.
- The diagonals of a parallelogram bisect each other.
- It is possible to create a tessellation with any parallelogram.
- The parallelogram is itself a special case of a trapezoid.
Vector spaces
In a vector space, addition of vectors is usually defined using the parallelogram law. The parallelogram law distinguishes Hilbert spaces from other Banach spaces.
Proof that diagonals bisect each other
To prove that the diagonals of a parallelogram bisect each other, first note a few pairs of equivalent angles:
Since they are angles that a transversal makes with parallel lines and .
Also, since they are a pair of vertical angles.
Therefore, since they have the same angles.
From this similarity, we have the ratios
Since , we have
- .
Therefore,
bisects the diagonals and .
Derivation of the area formula
The area formula,
can be derived as follows:
The area of the parallelogram to the right (the blue area) is the total area of the rectangle less the area of the two orange triangles. The area of the rectangle is
and the area of a single orange triangle is
Therefore, the area of the parallelogram is
See also
- Fundamental parallelogram
- Parallelogram of force
- Rhombus
- Synthetic geometry
- Gnomon (figure)
External links
- Parallelogram and Rhombus - Animated course (Construction, Circumference, Area)
- Eric W. Weisstein. Parallelogram. MathWorld
- Interactive Parallelogram —sides, angles and slope
- Area of Parallelogram at cut-the-knot
- National Parallogram Dealers WebsiteNational Parallelogram Dealers Association
- Equilateral Triangles On Sides of a Parallelogram at cut-the-knot
- Varignon and Wittenbauer Parallelograms by Antonio Gutierrez from "Geometry Step by Step from the Land of the Incas"
- Van Aubel's theorem Quadrilateral with four squares by Antonio Gutierrez from "Geometry Step by Step from the Land of the Incas"
- Parallelogram Quiz
- Definition and properties of a parallelogram with animated applet
- Interactive applet showing parallelogram area calculation interactive applet
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