The Devil's Triangle
Contents
Definition
For any triangle 
, let 
and 
be points on 
and 
respectively. Devil's Triangle Theorem states that if 
and 
, then ![$\frac{[DEF]}{[ABC]}=1-\frac{r(s+1)+s(t+1)+t(r+1)}{(r+1)(s+1)(t+1)}$](http://latex.artofproblemsolving.com/9/9/6/9968c2deab3db080b719289db31b415efae3ad10.png)
.
Proof
Proof 1
We have the following ratios:
.
Now notice that ![$[DEF]=[ABC]-([BDF]+[CDE]+[AEF])$](http://latex.artofproblemsolving.com/3/4/8/348cf5258a141132df51ae2e1ec60dd89e37d47f.png)
.
We attempt to find the area of each of the smaller triangles.
Notice that ![$\frac{[BDF]}{[ABC]}=\frac{BF}{AB}\times \frac{BD}{BC}=\frac{r}{(r+1)(t+1)}$](http://latex.artofproblemsolving.com/e/1/f/e1f4ef80cad3ed00c9d0bc6fb8b2f56ed0faca5f.png)
using the ratios derived earlier.
Similarly, ![$\frac{[CDE]}{[ABC]}=\frac{s}{(r+1)(s+1)}$](http://latex.artofproblemsolving.com/6/a/d/6ad09b527364466a3f92f10e3170e1600f3fd300.png)
and ![$\frac{[AEF]}{[ABC]}=\frac{t}{(s+1)(t+1)}$](http://latex.artofproblemsolving.com/f/b/d/fbd5231991313efc841a68b5bcbb6794dfec3004.png)
.
Thus, ![$\frac{[BDF]+[CDE]+[AEF]}{[ABC]}=\frac{r}{(r+1)(t+1)}+\frac{s}{(r+1)(s+1)}+\frac{t}{(s+1)(t+1)}=\frac{r(s+1)+s(t+1)+t(r+1)}{(r+1)(s+1)(t+1)}$](http://latex.artofproblemsolving.com/a/0/1/a012303031a664b7d67454d68942d25743528b37.png)
.
Finally, we have ![$\frac{[DEF]}{[ABC]}=\boxed{1-\frac{r(s+1)+s(t+1)+t(r+1)}{(r+1)(s+1)(t+1)}}$](http://latex.artofproblemsolving.com/5/e/a/5eaa6311987457d807a81db166673e522586049d.png)
.
Other Remarks
This theorem is a generalization of the Wooga Looga Theorem, which @RedFireTruck claims to have "rediscovered". The link to the theorem can be found here: https://artofproblemsolving.com/wiki/index.php/Wooga_Looga_Theorem
Essentially, Wooga Looga is a special case of this, specifically when 
.
