How many diagonals can be drawn from one vertex of nonagon?

Yes, as long as you have the TV hooked up to the Xbox through HDMI and the monitor hooked up using a VGA cable.

One.

Check the angles between either of the original rays and the bisector line. If they are equal, then the line is the exact bisector.

This is more a matter of intuition than proof. It's the subject of Euclid's Parallel Postulate. It's considered an axiom and has been the subject of much controversy for centuries.

The basic formula to find the number of diagonals in a polygon is n(n-3)/2.

It is the same relationship for any polygon, as expressed in the formula n(n-3)/2.

The perimeter equals the side length times the number of sides. In this case, multiply 21 by 9 and you will get a perimeter of 189.

Measure from one corner to the corner directly across from it to find the length of a diagonal.

This isn't possible without more information. If you knew the size of one of the angles, or the length of one side, you might be able to find the length...

Depending on what material you plan on using, you can approach various prototype makers that work with that material. Sometimes you can get buyers and investors to look at it,...