A method for solving proportions by multiplying the numerator of each fraction by the denominator of the other fraction. If a/b = c/d, then cross-multiplying gives ad = bc.
The term 'cross' refers to the X-pattern formed when drawing lines between the numerators and denominators being multiplied. This technique was formalized in European mathematics during the Renaissance as algebraic methods developed, though the underlying principle was known to ancient mathematicians.
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