人教版九年级(上)数学《圆》的一章中,有些定理和重要结论都有一个前提条件“在同圆或等圆中”.如教材83页定理:在同圆或等圆中,相等的圆心角所对的弧相等,所对的弦也相等.这里的前提条件“在同圆或等圆中”不能少,因为没有这个前提条件,结论就不成立了.但教材的85页圆周角定理(圆周角定理是:在 In the chapter of the 9th grade of the People’s Education Press (upper class), some of the theorems and important conclusions have a precondition of “in a circle or equal circle”. For example, the theorem of page 83: the same circle or etc. In the circle, the equal arcs of equal central angles are equal, and the strings they are equal are equal. The precondition “” in the same circle or equal circle cannot be less, because without this precondition, the conclusion is not established. The 85-page circumferential angle theorem of the textbook (Cyber ​​Theorem is: