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
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/mathFutoshiki (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/mathFinger 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
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
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