Solve for an unknown term in a proportion.
根据比例式 A : B = C : D，输入任意三项求解第四项。
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Proportion and cross-multiplication | a : b = c : d is equivalent to a · d = b · c | Euclid, Elements, Book VII |
| Simplifying a ratio | Divide every term by the greatest common divisor to get the simplest integer ratio | Euclid, Elements, Book VII (Euclidean algorithm) |
| Fourth proportional | d = b · c / a given a, b and c | Euclid, Elements, Book VII |
| Scale ratio | Scale = map distance / ground distance, often written 1 : N | ISO 80000-1:2009, Quantities and units - Part 1: General |
比例计算器根据比例的基本性质（内项积等于外项积）求解未知项，适用于各类按比例分配与换算场景。