Athens · c. 417-c. 369 BCE
Theaetetus
Theaetetus was Greek mathematician. He developed a classification of irrational magnitudes and contributed to the theory of regular solids.
Biographical and historical reference works document Theaetetus’s role and the mathematical work summarized on this page.
3-5, 6-8, 9-12, College
The person and the work
Biography
Theaetetus was Greek mathematician associated with Athens. He developed a classification of irrational magnitudes and contributed to the theory of regular solids.
Material associated with his work entered later books of Euclid’s Elements. Together, these contributions connect algebra and functions and geometry and measurement to the working problems, tools, and questions of the period.
Mathematical contribution
What this hero added or preserved
Core mathematical work
He developed a classification of irrational magnitudes and contributed to the theory of regular solids.
Method, teaching, or application
Material associated with his work entered later books of Euclid’s Elements.
Why this matters
The larger lesson
Theaetetus shows that mathematics grows through proof, calculation, observation, design, teaching, or careful records. The lasting lesson is that algebra and functions and geometry and measurement can turn a difficult question into a method another person can inspect and reuse.
Place the work in time
Timeline
- c. 417
Life and working period
Theaetetus worked as Greek mathematician in Athens.
- Legacy
Lasting mathematical connection
Material associated with his work entered later books of Euclid’s Elements.
Key idea
An equation balances two descriptions
Theaetetus’s work shows how symbols can represent an unknown quantity while preserving equality.
In 8x + 1 = 49, subtracting 1 and dividing by 8 gives x = 6.
Try it yourself
Solve the equation
Solve 8x + 1 = 49.
- Subtract 1 from both sides.
- Divide both sides by 8.
- Substitute the result to check.
Show the answer
x = 6.
Math at work
Scientists and astronomers use the same habits
Scientists and astronomers use algebra and functions to plan, compare, test, or communicate decisions. Theaetetus’s work provides a historical example of that connection.
Keep curiosity alive
A little-known fact
Material associated with his work entered later books of Euclid’s Elements.
Theaetetus matters because of only one famous formula or anecdote.
The documented record is broader: He developed a classification of irrational magnitudes and contributed to the theory of regular solids.
Keep exploring
Connected learning paths
Practice these ideas
- Pre-AlgebraBridge arithmetic and algebra with integers, ratios, and equations.
- Algebra IPractice equations, variables, functions, ratios, and inequalities.
- Algebra IIWork with quadratics, polynomials, systems, and sequences.
- GeometryExplore shapes, angles, area, perimeter, volume, and classification.
- Advanced GeometryPractice coordinate geometry, transformations, vectors, and solids.