無料レッスン50本、幼稚園から三角法まで
自分のやり方で数学を学ぶ
子ども・10代・大人のための無料でわかりやすい数学レッスン — 数え方から三角法まで全範囲をカバー。すべてのレッスンに採点される練習問題とインタラクティブ図があります。
学年で探す
低学年 (K–2)
- たし算とひき算
- 位取り
- 数えからかけ算へ
- 九九とわり算
- 分数と小数
- 周りの長さと面積
- 因数と素数
- 負の数と計算順序
- 変数と方程式
- 比とパーセント
- 角・三角形・円
- 統計の基礎
- グラフ・連立・二次方程式
- 多項式と因数分解
- ピタゴラスと三角法
- 確率と数え上げ
- 両方向の暗算
- 見積もりと文章題
- 2進数・浮動小数・剰余
四則演算 — 両方向で
Addition — carry right
The classic column method: ones first, carry to the left.
Addition — biggest first
Left-to-right addition: hundreds, then tens, then ones.
Subtraction — borrow right
The standard borrowing method, step by step.
Subtraction — left to right
Count down by chunks, or biggest column first.
Multiplication — standard way
Right-to-left multiplying with carrying.
Multiplication — biggest first
Partial products left to right — the mental-math way.
Division
Long division, short division, and chunking.
代数
Variables & expressions
Letters that hold numbers — evaluate and translate.
One-step equations
Undo one thing: the balance rule that starts algebra.
Two-step equations
Undo in reverse order — with step-by-step checking.
Inequalities
<, >, ≤, ≥ — and the one rule that flips the sign.
Exponents & powers
Repeated multiplication, the three rules, x⁰ and x⁻ⁿ.
Square roots
Perfect squares, estimating, and simplifying √12.
Polynomials
Terms, degrees, like terms, and distributing.
Factoring
GCF, x²+bx+c, and difference of squares.
Graphing lines
y = mx + b — slope and intercept on the plane.
Systems of equations
Substitution, elimination, and where lines meet.
Quadratic equations
Parabolas, zero-product, and the formula that always works.
幾何
Angles & lines
Acute to straight, complements, parallel lines.
Triangles
The 180° rule, families, and why bridges trust them.
Quadrilaterals
The four-sided family tree and the 360° rule.
Circles
Radius, diameter, π — C = πd and A = πr².
Perimeter & area
Around vs inside — with unit-square grids.
Surface area & volume
Wrapping vs filling — boxes, cylinders, unit³.
The coordinate plane
(x, y) addresses, quadrants, distance.
Transformations
Slide, flip, spin, scale on the grid.
Pythagorean theorem
a² + b² = c², triples, and the distance formula.
Similarity & congruence
Same shape, different size — scale factors.
数と構造
Place value & bases
Why 467 means 400 + 60 + 7 — and how binary works.
Fractions
What numerator & denominator mean, adding, comparing.
Decimals & percents
Place value past the point; everyday percent tricks.
Negative numbers
The number line, and why neg×neg = pos.
Order of operations
PEMDAS honestly — including the ambiguous case.
Factors & primes
Divisibility tricks and prime factorization.
比・データ・確率
Ratios & proportions
Comparing quantities and cross-multiplying.
Unit rates & percents
Per hour, per dollar — and the better-deal test.
Statistics basics
Mean, median, mode, range — and when the mean lies.
Spread & deviation
Variance and standard deviation as 'typical distance.'
Probability basics
0 to 1 — favorable/total, complements, AND/OR rules.
Counting principles
Permutations, combinations, factorials.
三角法
sin, cos, tan
SOH-CAH-TOA on right triangles — sides and angles.
The unit circle
(cos, sin) as a point — and the free identity.
思考力
Estimation & rounding
Front-end estimating and Fermi problems.
Word problems
Pólya's four-step method for attacking any problem.
Data, averages & graphs
Mean vs median — and how charts can politely lie.
科学と計算機の数学
Scientific notation & SI
Writing the universe's biggest and smallest numbers.
Measurement & error
Accuracy vs precision, significant figures, honest numbers.
Math inside computers
Binary, bits, floating point — and 0.1 + 0.2 ≠ 0.3.
Binary & hexadecimal
How computers count — $FF and 0x10 explained.
Floating point
Why 0.1 + 0.2 ≠ 0.3 — and the integer-cents trick.
Modular arithmetic
Clock math — MOD, parity tests, wrap-around indexing.
二つの向き、同じ答え
ほとんどの教科書は右から左への方法しか示しません。しかし熟練した暗算の人は逆に、大きい位から計算します。どちらが優れているわけでもなく、同じ算数を別の端から見ただけです。
なぜ二つの方法？
研究によれば、子どもが工夫する左から右への戦略は一般的で、位取りの理解を深めます。 →