Como nos triângulos semelhantes os lados homólogos são proporcionais, o resultado da divisão desses lados será um valor constante. Esse valor é chamado de razão de proporcionalidade.[br][br]Considere os triângulos ABC e EFG semelhantes, representados na figura abaixo:

Os lados a e e, b e g, c e f são homólogos, sendo assim, temos as seguintes proporções:[br][img width=115,height=38]data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAHMAAAAmCAYAAAD3AKSiAAAACXBIWXMAAA7EAAAOxAGVKw4bAAAABGJhU0UAAAAXQ/cXWQAAAyZJREFUeNrtW19klWEYf80ck4nJJJlIFzO7GJlJkkgymdndLueQLpOYzCQZM5Mk0cUkSUzORbKbzMxMRjKZZHTRxZE5TGaSHNbzc34f317fVGfrPM/79f748X3vzfd4nvd9/r3P51w4+C5schHB45TwQ1RDPjAofBHVUD/g0u4IN4Q/havCE0qy3BaOUZ6vwh/CBeExRf20CqeEZeqnJGyzaszXwgcUEIad5QnRAL79WTiakuee4mmFId8LJ4SHhQXhTeEVi4YcpgLTeCrsV5JnXdjlrUGJW0ryTHKjBwG4sDPe2qKSm21iJuujRcmYkKdi2aX+rgzA87aSLH3cXD6w2eYV5OkVLoWU/FS897OMEVou/0nG+g3huII8/Ux2gkGZbiwBlPZSSRbEphFvrcC6UyObPS38FJIx7wof070iTj4XzinJUmICdJHvx7k2oqifVWayzUzErtN7mcUYM9pxntINpdIE3x2gQatU5KCybjqYEFbpIYqK+UxdaI99FHXEtmaOENuaOQLamqN8RrxuWCNn5w/YSFiRZz9yJC3VFj5fiPs7XCAJ7Ba+crVmyr52TKN2804O5DloJG3NtzTof+nW8uJmk7YmOmIz0UmFjXRbc1p4LaokXKCteTX1/sYZ7zqFCMSyow34TskrQ3AFt+J0py1yhcvCTXKNtd+/wqbbffnhmAjNZ6xH/CVgONwu9fC9K6okXAy52FrLDdKttSCAwI5LaYw2fhGeV5TlkKuNNlYoD+R6qKTQklcXDls3JC5dcd0ykIoJa4rxaZGGayYxNoL7RK27TUwbdIRyKnEKbnlrmKUtKMiCC/KJjPV1pdR8r4lBs7XTN7f7QhprW07n5x1MsR/JkNHaxKBJ9LnsnuGygiwYoFoyptC9JgZNAnFy1lAJ8MyYQv3WmmlgwHjF0MbKqudg4KKSTH5rzTw+utoIIWJTG5Ohc0pZNSb0kk5LD8uSsmKplNVaM42TjFVVGraofDpR5+LXuXfCS6w3NZKxdiZkEQeEbqfTSrvP+nIymqA+dAofperJXmbVnUqZ9VA0Sf1oZfa4zUJ9zinMwISAX3iR/E6SQKt+AAAA0HRFWHRNYXRoTUwAPG1hdGggeG1sbnM9Imh0dHA6Ly93d3cudzMub3JnLzE5OTgvTWF0aC9NYXRoTUwiPjxtZnJhYz48bWk+YTwvbWk+PG1pPmU8L21pPjwvbWZyYWM+PG1vPj08L21vPjxtZnJhYz48bWk+YjwvbWk+PG1pPmc8L21pPjwvbWZyYWM+PG1vPj08L21vPjxtZnJhYz48bWk+YzwvbWk+PG1pPmY8L21pPjwvbWZyYWM+PG1vPj08L21vPjxtaT5rPC9taT48L21hdGg+yPIA5QAAAABJRU5ErkJggg==[/img][br]Onde k é a razão de proporcionalidade.