# Help on Angular Proof

1. Dec 17, 2005

### cujo_man

Hi
Here's the problem I'm stuck with. ABCDE is a regular pentagon. Point A is at the top and C and D are the two points at the bottom forming the commonly seen isosceles trianglein the midle. It then asks to find angle CAD (angle A). Since no lengths or other angles are given, can't i just make a line parallel to CD connecting with A.Then i could just do 180 / 5 to find angle A. i'm not sure if thats the way to do it. If anybody could help, thanks a lot.

2. Dec 17, 2005

### Tide

How do you know that all five of the angles at A are equal to each other?

3. Dec 18, 2005

### cujo_man

well, since no angles or lengths are given, theres no way to know. Thats why i'm stuck with this question.

4. Dec 18, 2005

### Tide

In that case, try this:

The vertices can be placed on a circle. Label the center of the circle O and construct line segments from O to C and to D. Angle COD is twice angle CAD.

Incidentally, this provides an answer to my earlier question. :)

5. Dec 19, 2005

### CarlB

It may be useful to go to this site:
http://en.wikipedia.org/wiki/Circle

and look up the "inscribed angle theorem". You've probably already covered it in class. It won't be enough to solve the problem, but it may be the push you need. Keep plugging.

Carl