 that which is equal to the equal is equal and this is also necessary. then now we work with the triangle and point to two lines from it and say that they are equal because they are two straight lines that come out from the center of a circle to its circumference and the third line is like one of them because
 أن المساوي للمساوي مساو وهذه ايضا ضرورية ثم الآن نشتغل بالمثلث ونشير الى خطين منه ونقول إنهما متساويان لانهما خطان مستقيمان خرجا من مركز دائرة الى محيطها والخط الثالث مثل لاحدهما لانه