What's missing from Khan Academy?

Writing single-digit numbers as numerals and words (kindergarten)

Identified by inspection

Kindergarten Math Course
by JoAnn's School

Congruent complements theorem (description and proof)

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congruent complements site:https://www.khanacademy.org/math/geometry

What is the Congruent Complements Theorem?
by Wrath of Math

Congruence Complements Theorem
by Math Teacher

Congruent supplements theorem (description and proof)

Identified by search
congruent supplements site:https://www.khanacademy.org/math/geometry

Congruence Supplements Theorem
by Math Teacher

Proof: The square root of any non-perfect square is irrational

Identified by inspection

Proof that the square root of ANY integer is irrational (besides perfect squares)
by Mu Prime Math

False proofs

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false proof site:https://www.khanacademy.org/math/
fallacious proof site:https://www.khanacademy.org/math/
find mistake site:https://www.khanacademy.org/math/

Proof that 1 = 2.
by John Hush

Proof that 0 = 1
by John Hush

A Proof that 0 = 1 (Can You Spot the Mistake?)
by patrickJMT

Water jug riddles

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water site:https://www.khanacademy.org/math

How not to Die Hard with Math
by Mathologer

Futoshiki puzzles

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futoshiki site:https://www.khanacademy.org/math
Futoshiki (fun puzzle with inequalities)
by Eddie Woo

Finger multiplication tricks (and their correctness proofs)

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finger multiply site:https://www.khanacademy.org/math
finger multiplication site:https://www.khanacademy.org/math
finger product site:https://www.khanacademy.org/math
Finger multiplication on steroids
by Mathologer

Two proofs: Area of dodecagon

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dodecagon site:https://www.khanacademy.org/math

Area of a Regular Dodecagon (visual proof)
by Micro Visual Proofs

Area of a Regular Dodecagon II (visual proof)
by Micro Visual Proofs

Po-Shen Lo method

Identified by inspection

a new algebra 2 quadratic formula
by Dr Peyam Show

Proof: Inverse Pythagorean theorem

Identified by search
inverse pythagorean site:https://www.khanacademy.org/math

Pythagorean Theorem and Its Inverse
by blackpenredpen

Palindromic polynomials

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palindrome site:https://www.khanacademy.org/math
palindromic site:https://www.khanacademy.org/math

Magic Polynomials
by Dr Peyam Show

Division as repeated subtraction

Identified by search
division repeated subtraction site:https://www.khanacademy.org/math/
divide subtract site:https://www.khanacademy.org/math/

and inspection

North School Math - Repeated Sub Strategy
Londonderry, NH North Elementary School

KenKen puzzles

Identified by search
kenken site:https://www.khanacademy.org/math

Tom Davis, "KenKen"
by Math Circle Network

River crossing puzzles

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river site:https://www.khanacademy.org/math

River Crossings

Patterns in Pascal's triangle

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pascal's triangle site:https://www.khanacademy.org/math

nCr Notation (2 of 2: The mystery of the prime rows in Pascal's Triangle)
by Eddie Woo

Viviani's theorem (description and proof)

Identified by search
viviani site:https://www.khanacademy.org/math
viviani's site:https://www.khanacademy.org/math

First visual proof:

Geometry: Viviani's theorem | Visualization + Proof |
by Think Twice

Second visual proof:

Viviani's theorem
by Wikipedia

Proof: Exactly 3 regular tessellations

Identified by inspection
tessellation site:https://www.khanacademy.org/math
tiling site:https://www.khanacademy.org/math

Geometry 9.6b, Tessellations, Regular, Semi-regular, or neither
by JoAnn's School

Intro to triangular numbers

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triangular numbers site:https://www.khanacademy.org/math

What are Triangular Numbers?
by Learning at Home with Mr B

Visual proof: Sum of the first n triangular numbers

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triangular numbers site:https://www.khanacademy.org/math

Sum of n Triangular Numbers - Animation
by Mathocube

Ptolemy's theorem

Identified by inspection


How to Prove Ptolemy's Theorem for Cyclic Quadrilaterals
by ProfOmarMath

Visual proof:

A Beautiful Proof of Ptolemy's Theorem.
by MathyJaphy

Fuhrmann's theorem

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fuhrmann site:https://www.khanacademy.org/math

fuhrmann's theorem
by Wilson Stother

British flag theorem

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british flag site:https://www.khanacademy.org/math

British Flag Theorem
by AoPS

0.999... = 1

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0.999 site:https://www.khanacademy.org/math

Does 0.99999.. equal one or does it not?
by Mathematical Association of America

Proof: Converse of the Pythagorean theorem

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converse pythagorean site:https://www.khanacademy.org/math

Proof of the Converse of Pythagoras' Theorem
by KeysToMaths1

Circumcenter theorem

This topic occurs in "Geometry (all content)," but belongs in "High school geometry."

Circumcenter of a triangle
by Khan Academy

Expanding and factoring with algebra tiles

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algebra tiles site:https://www.khanacademy.org

and inspection

Factoring Simple Trinomials Using Algebra Tiles
by MrsALovesMath

6.5 Factoring Trinomials with Algebra Tiles
by Algebra 1 - Ladue Mathematics


Identified by search
t-results site:https://www.khanacademy.org/math

Search "eddie woo t-results" on YouTube.

Multiplying two binomials using algebra tiles

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Visual Proof for Difference of Squares a²-b² by Eddie Woo

Adding and subtracting integers using two color counters

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integers counters site:https://www.khanacademy.org/math

Search "mathispower4u add subtract integers counters" on YouTube.

Children's book recommendations

Proof: Exactly 5 Platonic solids

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platonic site:https://www.khanacademy.org/math

Mysterium Cosmographicum
by D!NG

Solving factorial equations

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factorial equation site:https://www.khanacademy.org/math

How to solve equations with Factorials
by Anil Kumar

Divisibility rules for 7 and 13
Proving the divisibility rules for 7 and 13
Sum of first n odd numbers?
Crossed ladders theorem
Most visual proofs
Visual proof: Difference of cubes
Most topics concerning even and odd functions
Sophie Germain's identity
Gauss's trick for tetrahedral numbers
Hard problems
Variations on the Monty Hall problem
Billiard bouncing puzzles?
Golf puzzles?
Area mazes?
Geometry puzzles?
Textbook recommendations
Nim-like games
Math circles
St. Petersburg paradox?
Boy-girl paradox?
Converse of each corresponding angle theorem
Converse of the Pythagorean theorem
Triangle midsegment theorem
Solving nested absolute value equations?
Finding all the factors of a number using Cuisenaire rods
Finding the GCD or LCM of two or more numbers, using Cuisenaire rods