[子部,天文算法类,算书之属,几何论约钦定四库全书几何论约卷四之首柘城杜知耕撰界説七则一界此直线形居他直线形内此直线形为他直线形内切形二界此直线形居他直线形外此直线形为他直线形外切形三界圜内直线形以各角切圜界为圜内切形四界圜外直线形以各边切圜界为圜外切形五界直线形内圜圜界切直线形之各边为形内切圜六界直线形外圜圜界切直线形之各角为形外切圜七界直线之两端各抵圜为合圜线如甲乙丙丁两线俱为合圜线而戊己辛庚两线或至界或不至界或俱不至界皆不得为合圜线