Area
Introduction
In mathematics, area refers to the size of the region that a two-dimensional figure occupies.
Generally, contest problems are only concerned with finding the area of regions bounded by straight line segments, circles, or sometimes even ellipses.
One can find the area of even more complex regions via the use of calculus.
Rectangles are the most basic of figures to derive the area of. It makes sense that the area of a rectangle with lenght l and width w is simply 
.
Once we know the area of a rectangle we can easily find the area of a triangle by just noting that if our triangle has base b and height h then the rectangle with length b and width h has exactly twice as much area as the original triangle. Thus, the area of a triangle is
We can now use find the area of any polygon by breaking it up into triangles.
Notation
Some popular notations for area include:
- A
- When there are multiple regions involved in a problem subscripts can be added to the A such as
or 
.
- When there are multiple regions involved in a problem subscripts can be added to the A such as
- Brackets around the name of the region, e.g.
![$[ABC]$](//latex.artofproblemsolving.com/d/3/3/d33cc80fa8f093e155c5be46d2e5d9da3d7e1ef5.png)
.
or 
.![$[ABC]$](http://latex.artofproblemsolving.com/d/3/3/d33cc80fa8f093e155c5be46d2e5d9da3d7e1ef5.png)
.Area Formulas
In the following formulas, assume we have 
with 
, base b, height h, circumradius R and inradius r. Here are important formulas for the area of a triangle.
- [ABC]=
- For an equilateral triangle with side length s:
![$[ABC] = \frac{s^2\sqrt{3}}4$](http://latex.artofproblemsolving.com/c/a/2/ca233034e99b30bb433156600d48ab30ecfe12bc.png)
