### Shape and Territory

If for any triangle ABC tan(A - B) + tan(B - C) + tan(C - A) = 0 what can you say about the triangle?

### Napoleon's Hat

Three equilateral triangles ABC, AYX and XZB are drawn with the point X a moveable point on AB. The points P, Q and R are the centres of the three triangles. What can you say about triangle PQR?

### The Root Cause

Prove that if a is a natural number and the square root of a is rational, then it is a square number (an integer n^2 for some integer n.)

# Tree Graphs

##### Stage: 5 Challenge Level:

A graph is a network of points and lines. The points are called vertices and the lines which connect them are called edges. Each edge has a vertex at each end. The branch of mathematics called graph theory studies the properties of various kinds of graphs.

A connected graph is a graph in which we can get from any vertex to any other by travelling along the edges. A tree is a connected graph with no closed circuits (or loops).

Prove that every tree has exactly one more vertex than it has edges.