National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Logic Definitions: Inductive & Deductive
This site explains what deductive and inductive reasoning is. The site also explains abductive reasoning.
Maths Challenge
Maths challenge.net: Odd Vertices
A "three star geometry problem" requiring knowledge of how to complete geometric proofs. The problem explains what the order of a vertex is and asks for the proof of a generalization.
Math Planet
Math Planet: Geometry: Conjecture
Conjecture is explained through examples and a video lesson. [0:41]
Texas Instruments
Texas Instruments: Numb3 Rs: The Missing City
Based off of the hit television show NUMB3RS, this lesson has students examine maps of a crime spree in order to figure out if any locations have been unreported. The lesson is not math-intensive and instead forces students to use...