Seja um circunferência de centro O, sobre a qual tomamos dois pontos distintos, A e B. A circunferência fica dividida em duas partes, cada uma das quais é um[b] arco de[/b] [b]circunferência[/b].[br][justify] Observe na construção abaixo, que existem dois arcos determinados por A e B.[br][br] Movimente o ponto A de forma que ele continue entre X e Y.[br] Movimente o ponto B de forma que ele continue entre X e Y.[/justify][justify][/justify]
Quando não houver dúvidas em relação ao arco ao qual nos referimos, podemos escrever simplesmente [img]data:image/png;base64,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[/img] para representar o arco com extremidades A e B.
[justify] Vejamos agora dois casos particulares:[br]1) Se a A e B são simétricos em relação ao centro O, o segmento AB é um diâmetro e cada um dos arcos determina uma semicircunferência e é chamado [b]arco de meia-volta[/b].[br] Movimente o ponto B.[/justify]
2) No caso de A coincidir com B, dois arcos são determinados. Um deles é o [b]arco de uma volta[/b] e o outro, o [b]arco nulo[/b].[br] Movimente o ponto A.
Observe que todo arco [img]data:image/png;base64,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[/img] corresponde um ângulo central, isto é, um ângulo cujo vértice é o centro da circunferência.[br][justify] Movimente o ponto A ou o ponto B.[/justify]