Inradius
The inradius of a polygon is the radius of its incircle (assuming an incircle exists). It is commonly denoted 
.
![[asy] pathpen = linewidth(0.7); pair A=(0,0),B=(4,0),C=(1.5,2),I=incenter(A,B,C),F=foot(I,A,B); D(MP("A",A)--MP("B",B)--MP("C",C,N)--cycle); D(CR(D(MP("I",I,SW)),inradius(A,B,C))); D(F--I--foot(I,B,C)--I--foot(I,C,A)); D(rightanglemark(I,F,B,5)); MP("r",(F+I)/2,E); [/asy]](http://latex.artofproblemsolving.com/3/a/d/3ad1b30f1350fc22f90ce7a79f2567025d4fe4c0.png)
A Property
- If
has inradius 
and semi-perimeter 
, then the area of is 
. This formula holds true for other polygons if the incircle exists.
has inradius 
, then the 
. This formula holds true for other polygons if the incircle exists.Proof
Add in the incircle and drop the altitudes from the incenter to the sides of the triangle. Also draw the lines 
, and 
. After this AB, AC, and BC are the bases of 
, and 
respectively. But they all have the same height(the inradius), so $[ABC}=\frac{(a+b+c)r}{2} =rs$ (Error compiling LaTeX. Unknown error_msg).
Problems
- Verify the inequality
. - Verify the identity
(see Carnot's Theorem). - 2007 AIME II Problems/Problem 15
.
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