Isosceles Triangles
Stage: 3 Challenge Level: 
Isosceles triangles have two equal sides.
The area of a triangle is half the base times the height.
Any of the three vertices of the triangle can be at $(20, 20)$.
There are sets of identical (congruent) triangles with an area of
$9$cm$^2$.
How many triangles are there in each set? How many sets?