# inscribe circle

## Quadrilateral Circumscribing a Circle

Quadrilateral circumscribing a circle (also called tangential quadrilateral) is a quadrangle whose sides are tangent to a circle inside it.

Area,

Where r = radius of inscribed circle and s = semi-perimeter = (a + b + c + d)/2

**Derivation for area**

## The Regular Polygon

Rhombus is a quadrilateral with all sides equal (equilateral). Rectangle is a quadrilateral with all included angles are equal (equiangular). Square is both equilateral and equiangular, thus square is a regular polygon. Regular polygons are polygons with all sides equal and all included angles equal. Meaning, regular polygons are both equilateral and equiangular.

## Derivation of Formula for Radius of Incircle

The radius of incircle is given by the formula

where A_{t} = area of the triangle and s = semi-perimeter.

## Centers of a Triangle

This page will define the following: incenter, circumcenter, orthocenter, centroid, and Euler line.

**Incenter**

Incenter is the center of the inscribed circle (incircle) of the triangle, it is the point of intersection of the angle bisectors of the triangle.

The radius of incircle is given by the formula

where A_{t} = area of the triangle and s = ½ (a + b + c). See the derivation of formula for radius of incircle.

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