What's missing from Khan Academy?


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

Identified by inspection
https://www.khanacademy.org/math/cc-kindergarten-math

Kindergarten Math Course
by JoAnn's School


Congruent complements theorem (description and proof)

Identified by search
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
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:irrational-numbers#x2f8bb11595b61c86:proofs-concerning-irrational-numbers

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


False proofs

Identified by search
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

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

How not to Die Hard with Math
by Mathologer


Futoshiki puzzles

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

Finger multiplication tricks (and their correctness proofs)

Identified by search
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

Identified by search
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
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratic-functions-equations

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

Identified by search
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
https://www.khanacademy.org/math/arithmetic-home/multiply-divide#division-intro

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

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

River Crossings
by JRMF


Patterns in Pascal's triangle

Identified by search
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

Identified by search
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

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

Sum of n Triangular Numbers - Animation
by Mathocube


Ptolemy's theorem

Identified by inspection
https://www.khanacademy.org/math/engageny-geo/geo-5/geo-5e/a/geo-5e

Proof:

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

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

fuhrmann's theorem
by Wilson Stother


British flag theorem

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

British Flag Theorem
by AoPS


0.999... = 1

Identified by search
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

Identified by search
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

Identified by search
algebra tiles site:https://www.khanacademy.org

and inspection
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring

Factoring Simple Trinomials Using Algebra Tiles
by MrsALovesMath

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


t-results

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

Search "eddie woo t-results" on YouTube.


Multiplying two binomials using algebra tiles

Identified by inspection
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring#x2f8bb11595b61c86:multiply-binomial
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring#x2f8bb11595b61c86:special-product-binomials

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


Adding and subtracting integers using two color counters

Identified by search
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

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

Mysterium Cosmographicum
by D!NG


Solving factorial equations

Identified by search
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