100 free lessons · Worked examples and practice
Undergraduate math
Explore selected university topics across proof, algebra, geometry, analysis, and probability. Each lesson offers a focused entry point with an explicit problem and a worked argument you can reconstruct.

Your study guide
What you will explore.
- Read and construct mathematical arguments
- Connect algebraic, geometric, and analytic viewpoints
- Apply models while checking their hypotheses
Before you begin
There is no single linear prerequisite chain across all 100 lessons. Choose a subject you know, read its prerequisites, and review the necessary calculus, linear algebra, or proof techniques first.
Review Grade 12How to study these lessons
Rewrite the argument without looking at the solution. Identify where each assumption enters, and try a small example or counterexample to test the boundaries of the claim.
Work at your own pace. Open a hint when you need it, then try again before revealing the complete explanation.
Grade labels are starting-point guides; school sequences vary. University lessons introduce selected topics and assume the prerequisites listed on each page.
Practice with a moving example
650 guided practice chapters for Undergraduate.
Try a distinct question, open a hint, compare the worked solution, and explore its interactive animation. These scenario chapters give you more practice within this level’s topics.
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Teaching videos, chapter by chapter.
Teaching recordings for this level are being prepared. Browse the chapter index to see video availability and open the written lessons that are ready now.
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Your Undergraduate lesson collection.
The lessons appear in their suggested study order. Choose the first lesson for a starting point, or use the search and subject filters to review a particular skill. Read the prerequisites whenever you change topics.
Showing 12 of 100 matching lessons · All free
AlgebraUndergraduate · Advanced 16 min
Why the order of quantifiers changes a claim
Distinguish a uniform choice from a choice that depends on the input.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Build a strong-induction postage argument
Use several base cases to support a recurrence that moves by four.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Compress integers into residue classes
Verify an equivalence relation and perform well-defined arithmetic on classes.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Recover a greatest common divisor as a linear combination
Run Euclid's algorithm and reverse it to obtain Bézout coefficients.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Undo multiplication modulo a prime
Find a modular inverse through a Bézout identity rather than trial division.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Combine two independent remainder schedules
Solve simultaneous congruences and identify the repetition period.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Reuse elimination with an LU factorization
Separate elimination from back substitution to solve repeated linear systems.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Find all invisible inputs of a linear map
Parameterize a nullspace and connect free variables to lost information.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Describe the same vector in another coordinate system
Solve for coordinates relative to a nonstandard basis.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Turn differentiation into a matrix
Represent an operator using its action on basis vectors.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Separate a signal from its perpendicular residual
Compute a least-distance projection onto a line.
Read, practice, and reflect
AlgebraUndergraduate · Advanced 16 min
Create perpendicular directions without changing the span
Orthogonalize independent vectors by subtracting a projection.
Read, practice, and reflect