Maths Challenge
Maths challenge.net: Unique Circle Equal Angles
A "three star geometry problem" - requiring the knowledge of biconditional statements and how to prove such problems.
Wolfram Research
Wolfram Math World: Parallel Planes
MathWorld.com gives a definition of parallel planes along with an if and only if (iff) statement to determine if two planes are parallel. The understanding of the dot product and/or cross product is necessary to utilize the iff...