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Handout
Wikimedia

Wikipedia: Theorem

For Students 9th - 10th
Wikipedia offers a detailed description of a theorem, or a statement that can be proven true. Includes many terms related to theorems, also.
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Handout
Wolfram Research

Wolfram Math World: Conjecture

For Students 9th - 10th
This MathWorld site gives the definition of conjecture with links to related sites.