## 2007年10月18日 星期四

### 圓周角是圓心角的兩倍的逆定理：解答

「圓周角是圓心角的兩倍」這定理是可逆的，以下是其中一個逆定理的版本：

#### 4 則留言:

Angela 提到...

For the first "converse", point A could lie on any circle whose radius is the same as the circle with O as centre and passes through B and C.

Similarly, for the second "converse", point O could be the centre of another circle as well... I think locus of O is a circle in the plane perpandicular to the plane of the original circle and contains all the perpandicular bisectors of BC.

Is there a more accurate version of converse of 'angle at centre is double of angle at circumference'?

Kahoo 提到...

Oh, you are right. Well, first of all, we shall assume that all points are on the same plane, a "normal assumption" when we are doing plane geometry. Under this assumption, there are actually exactly two circles for both versions, being symmetric about BC. In order that the converses hold, we need A and O to be on the same side of BC. To be precise, the two versions should be as follows:

Version 1
Let O be the centre of a circle and B, C be two points on the circumference. If A lies on the same side of BC as O and ∠BOC = 2∠BAC, then A also lies on the circumference of the circle.

Version 2
Let A, B, C be three points on a circle. If ∠BOC = 2∠BAC, O lies on the perpendicular bisector of BC and O lies on the same side of BC as A, then O is the centre of the circle.

Version 2 doesn't very much look like a "converse". :P

Angela 提到...

What if ∠BAC is obtuse?

Kahoo 提到...

Oh, I overlooked that again. There are two ways to fix the problem. (Only Version 2 needs revision because in Version 1, ∠BOC is assumed to mean the non-reflex angle.) One is to further digress into the cases where ∠BAC is acute or obtuse, but that will further complicate the "converse". The other is to add the restriction that ABC (in this order) is a minor arc of the circle, discarding the case when ∠BAC is obtuse.