Online tools for practising rhythm and music theory. Log in to record your scores. Teachers can create exercises for students. Free and mobile friendly.

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4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is a square number, the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures.

Four fours is a mathematical puzzle, the goal of which is to find the simplest mathematical expression for every whole number from 0 to some maximum, using only common mathematical symbols and the digit four. No other digit is allowed.

Little rhythm apps for fun and practice. Metronomes, generators, a drum machine. Tools and trainers for learning rhythm.

FOUR翻译：（数字）4, 四人赛艇队;四人赛艇, （板球）得四分的一击（球被击到边界上）。 了解更多。

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Use exactly four 4's to form every integer from 0 to 50, using only the operators +, −, ×, /, (brackets), . (decimal point), √ (square root) and ! (factorial). Example: 0 = 44 − 44

四平方和定理：每个自然数可表示成最多4个平方数的和; 正四面体是最小面数的正多面体。 在一个平面的地图上，一定可以用四种颜色来填每个区域而相邻的区域颜色不相同，即四色定理。 最小的非循环群有四个元素，即克萊因四元群。4亦是最小的非簡單群的 ...