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Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat. It is the two-dimensional analog of the length of a curve (a one-dimensional concept) or the volume of a solid (a three-dimensional concept).
The area of a shape can be measured by comparing the shape to squares of a fixed size. In the International System of Units (SI), the standard unit of area is the square metre (m), which is the area of a square whose sides are one metre long. A shape with an area of three square metres would have the same area as three such squares. In mathematics, the unit square is defined to have area one, and the area of any other shape or surface is a dimensionless real number.
There are several well-known formulas for the areas of simple shapes such as triangles, rectangles, and circles. Using these formulas, the area of any polygon can be found by dividing the polygon into triangles. For shapes with curved boundary, calculus is usually required to compute the area. Indeed, the problem of determining the area of plane figures was a major motivation for the historical development of calculus.
For a solid shape such as a sphere, cone, or cylinder, the area of its boundary surface is called the surface area. Formulas for the surface areas of simple shapes were computed by the ancient Greeks, but computing the surface area of a more complicated shape usually requires multivariable calculus.
Area plays an important role in modern mathematics. In addition to its obvious importance in geometry and calculus, area is related to the definition of determinants in linear algebra, and is a basic property of surfaces in differential geometry. In analysis, the area of a subset of the plane is defined using Lebesgue measure, though not every subset is measurable. In general, area in higher mathematics is seen as a special case of volume for two-dimensional regions.
Formal definition
See also: Jordan measureAn approach to defining what is meant by area is through axioms. For example, we may define area as a function from a collection of special kind of plane figures (termed measurable sets) to the set of real numbers which satisfies the following properties:
For all in , () ≥ 0.If and are in then so are ∪ and ∩ , and also (∪) = () + () − (∩).If and are in with ⊆ then − is in and (−) = () − ().If a set is in and is congruent to then is also in and () = ().Every rectangle is in . If the rectangle has length and breadth then () = hk.Let be a set enclosed between two step regions and . A step region is formed from a finite union of adjacent rectangles resting on a common base, i.e. ⊆ ⊆ . If there is a unique number such that () ≤ c ≤ () for all such step regions and , then () = .It can be proved that such an area function actually exists. (See, for example, Elementary Geometry from an Advanced Standpoint by Edwin Moise.)
Units
Every unit of length has a corresponding unit of area, namely the area of a square with the given side length. Thus areas can be measure in square metres (m), square centimetres (cm), square millimetres (mm), square kilometres (km), square feet (ft), square yards (yd), square miles (mi), and so forth. Algebraically, these units can be thought of as the squares of the corresponding length units.
The SI unit of area is the square metre, which is considered an SI derived unit.
Conversions
The conversion between two square units is the square of the conversion between the corresponding length units. For example, since
1 foot = 12 inches,the relationship between square feet and square inches is
1 square foot = 144 square inches,where 144 = 12 = 12 × 12. Similarly:
1 square kilometer = 1,000,000 square meters1 square meter = 10,000 square centimetres = 1,000,000 square millimetres1 square centimetre = 100 square millimetres1 square yard = square feet1 square mile = 3,097,600 square yards = 27,878,400 square feetIn addition,
1 square inch = 6.4516 square centimetres1 square foot = 0.09290304 square metres1 square yard = 0.83612736 square metres1 square mile = 2.589988110336 square kilometresOther units
See also: Category:Units of areaThere are several other common units for area. The are was the original unit of area in the metric system, with
1 are = 100 square metresThough the are has fallen out of use, the hectare is still commonly used to measure land:
1 hectare = 100 ares = 10,000 square metres = 0.01 square kilometresOther uncommon metric units of area include the tetrad, the hectad, and the myriad.
The acre is also commonly used to measure land areas, where
1 acre = 4,840 square yards = 43,560 square feet.An acre is approximately 40% of a hectare.
On the atomic scale, area is measured in units of barns, such that,
1 barn = 10 square meters.The barn is commonly used in describing the cross sectional area of interaction in nuclear physics.
Basic area formula
Rectangles
The most basic area formula is the formula for the area of a rectangle. Given a rectangle with length and , the formula for the area is
= lw (rectangle).That is, the area of the rectangle is the length multiplied by the width. As a special case, the area of a square with side length is given by the formula
= (square).The formula for the area of a rectangle follows directly from the basic properties of area, and is sometimes taken as a definition or axiom. On the other hand, if geometry is developed before arithmetic, this formula can be used to define multiplication of real numbers.
Dissection formulae
Most other simple formulae for area follow from the method of dissection. This involves cutting a shape into pieces, whose areas must sum to the area of the original shape.
For an example, any parallelogram can be subdivided into a trapezoid and a right triangle, as shown in figure to the left. If the triangle is moved to the other side of the trapezoid, then the resulting figure is a rectangle. It follows that the area of the parallelogram is the same as the area of the rectangle:
= bh (parallelogram).However, the same parallelogram can also be cut along a diagonal into two congruent triangles, as shown in the figure to the right. It follows that the area of each triangle is half the area of the parallelogram:
(triangle).Similar arguments can be used to find area formulae for the trapezoid and the rhombus, as well as more complicated polygons.
Circles
The formula for the area of a circle is based on a similar method. Given a circle of radius , it is possible to partition the circle into sectors, as shown in the figure to the right. Each sector is approximately triangular in shape, and the sectors can be rearranged to form and approximate parallelogram. The height of this parallelogram is , and the width is half the circumference of the circle, or π. Thus, the total area of the circle is × π, or π:
= π (circle).Though the dissection used in this formula is only approximate, the error becomes smaller and smaller as the circle is partitioned into more and more sectors. The limit of the areas of the approximate parallelograms is exactly π, which is the area of the circle.
This argument is actually a simple application of the ideas of calculus. In ancient times, the method of exhaustion was used in a similar way to find the area of the circle, and this method is now recognized as a precursor to integral calculus. Using modern methods, the area of a circle can be computed using a definite integral:
Surface area
Archimedes showed that the surface area and volume of a sphere is exactly 2/3 of the area and volume of the surrounding cylindrical surface.Most basic formulae for surface area can be obtained by cutting surfaces and flattening them out. For example, if the side surface of a cylinder (or any prism) is cut lengthwise, the surface can be flattened out into a rectangle. Similarly, if a cut is made along the side of a cone, the side surface can be flattened out into a sector of a circle, and the resulting area computed.
The formula for the surface area of a sphere is more difficult: because the surface of a sphere has nonzero Gaussian curvature, it cannot be flattened out. The formula for the surface area of a sphere was first obtained by Archimedes in his work On the Sphere and Cylinder. The formula is
= 4πr (sphere).where is the radius of the sphere. As with the formula for the area of a circle, any derivation of this formula inherently uses methods similar to calculus.
List of formulae
The above calculations show how to find the area of many common shapes.
The area of irregular polygons can be calculated using the "Surveyor's formula".
Additional formulae
Areas of 2-dimensional figures
a triangle: (where is any side, and is the distance from the line on which lies to the other vertex of the triangle). This formula can be used if the height is known. If the lengths of the three sides are known then Heron's formula can be used: where , , are the sides of the triangle, and is half of its perimeter. If an angle and its two included sides are given, the area is where C is the given angle and a and b are its included sides. If the triangle is graphed on a coordinate plane, a matrix can be used and is simplified to the absolute value of . This formula is also known as the shoelace formula and is an easy way to solve for the area of a coordinate triangle by substituting the 3 points (x,y), (x,y), and (x,y). The shoelace formula can also be used to find the areas of other polygons when their vertices are known. Another approach for a coordinate triangle is to use Infinitesimal calculus to find the area.a simple polygon constructed on a grid of equal-distanced points (i.e., points with integer coordinates) such that all the polygon's vertices are grid points: , where is the number of grid points inside the polygon and is the number of boundary points. This result is known as Pick's theorem.Area in calculus
The area between a positive-valued curve and the horizontal axis, measured between two values and (>) on the horizontal axis, is given by the integral from to of the function that represents the curve.The area between the graphs of two functions is equal to the integral of one function, (), minus the integral of the other function, ().An area bounded by a function = (θ) expressed in polar coordinates is .The area enclosed by a parametric curve with endpoints is given by the line integrals(see Green's theorem) or the -component of
Surface area of 3-dimensional figures
cone: , where is the radius of the circular base, and is the height. That can also be rewritten as where is the radius and is the slant height of the cone. is the base area while is the lateral surface area of the cone.cube: , where is the length of an edge.cylinder: , where is the radius of a base and is the height. The can also be rewritten as d, where is the diameter.prism: 2B + Ph, where is the area of a base, is the perimeter of a base, and is the height of the prism.pyramid: , where is the area of the base, is the perimeter of the base, and is the length of the slant.rectangular prism: , where is the length, is the width, and is the height.General formula
The general formula for the surface area of the graph of a continuously differentiable function where and is a region in the xy-plane with the smooth boundary:
Even more general formula for the area of the graph of a parametric surface in the vector form where is a continuously differentiable vector function of :
Minimization
Given a wire contour, the surface of least area spanning ("filling") it is a minimal surface. Familiar examples include soap bubbles.
The question of the filling area of the Riemannian circle remains open.
