Division by zero
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In mathematics, division by zero, division where the divisor (denominator) is zero, is a unique and problematic special case. Using fraction notation, the general example can be written as , where is the dividend (numerator).
Handheld calculators, such as this TI-86, typically halt and display an error message after an attempt to divide by zero.
Multiplication
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Multiplication is one of the four elementary mathematical operations of arithmetic, with the other ones being addition, subtraction, and division. The result of a multiplication operation is called a product.
4 × 5 = 20. The large rectangle is made up of 20 squares, each 1 unit by 1 unit.
The Educated Monkey—a tin toy dated 1918, used as a multiplication "calculator". For example: set the monkey's feet to 4 and 9, and get the product—36—in its hands.