1. A rectilineal figure is said to be inscribed in a rectilineal figure when the respective angles of the inscribed figure lie on the respective sides of that in which it is inscribed.
I. FIGVRA rectilinea in figura rectilinea inscribi dicitur, cum singuli eius figuræ, quæ inscribitur, anguli singula latera eius, in qua inscribitur, tangunt.
第一界
直線形。居他直線形內。而此形之各角。切他形之各邊。為形內切形。
此卷將論切形在圜之內、外。及作圜在形之內、外。故解形之切在形內、及切在形外者。先以直線形為例。如前圖丁戊己角形之丁、戊、己、三角。切甲乙丙角形之甲乙、乙丙、丙甲、三邊。則丁戊己為甲乙丙之形內切形。如後圖。癸子丑角形。難癸、子、兩角。切庚辛壬角形之庚辛、壬庚、兩邊。而丑角、不切辛壬邊。則癸子丑、不可謂庚辛壬之形內切形。