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Definition

Proportion

A proportion is an equation of the form ab=cd\frac{a}{b} = \frac{c}{d}, where b0b \neq 0 and d0d \neq 0, asserting that two rational expressions are equal. This equation is read as "aa is to bb, as cc is to dd." For instance, 12=48\frac{1}{2} = \frac{4}{8} is a proportion — read "1 is to 2 as 4 is to 8" — because both fractions represent the same value. Proportions frequently appear in applications that involve scaling a known ratio to a larger or smaller context; when an unknown quantity occupies any of the four positions, it can be found by solving the equation. Because a proportion is a rational equation, it is solved by multiplying both sides by the LCD to clear the fractions and then solving the resulting equation. As with all rational expressions, any variable values that would make either denominator equal zero must be excluded.

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Updated 2026-05-01

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