[b]Definition:[/b] Given 4 integer numbers, [math]a,b,c[/math] and [math]d[/math], where [math]b[/math], and [math]d[/math] are nonzero, then ratios [math]a:b[/math] and [math]c:d[/math] are said to be a [b]proportion [/b]if and only if [math]ad=bc[/math].[br][br][b]Notation: [/b]Under the previous conditions, whenever ratios a:b and c:d are proportional is often written as[br][list][*][math]a:b::c:d[/math], or[/*][*][math]\frac{a}{b}=\frac{c}{d}[/math].[/*][/list][br][b]Remark:[/b] Notice how being two ratios being a proportion is just another way to say that the corresponding fractions are equivalent: [math]\frac{a}{b}=\frac{c}{d}[/math] if and only if [math]ad=bc[/math].