diagonals

[shahar]

Is there a proof this if I had two diagonals of quadrilateral I know what is it?
If I had only 2 diagonals of pentagon I can also or I need 3?
Is there a polygon that I have minimum diagonals but not all of them I will know what it is?

 

[Dr.Peterson]

As I understand this, you are asking,

1. Is it true that the lengths of the two diagonals of a quadrilateral are sufficient to determine that quadrilateral. [NO]
2. How many diagonals of a pentagon are sufficient to determine it?
3. Something I can't interpret.

Before asking whether something can be proved, you should at least experiment with it, to see whether it seems likely to be true. Did you make any attempt to draw two different quadrilaterals whose diagonals are the same?

Similarly, can you make a guess, and support it with examples, as to whether 2 or 3 diagonals are enough to determine a pentagon? Just imagine arranging 2 or 3 sticks, and see whether moving them would change the resulting pentagon.

Then try to restate your third question.

 

[khansaheb]

Is there a proof this if I had two diagonals of quadrilateral I know what is it?

No..

you need more parameters to ascertain the exact "quadrilateral".

You should know that three independent parameters for generalized triangle (ASA or SSS) are needed to prove uniqueness.

 

[shahar]

No..

you need more parameters to ascertain the exact "quadrilateral".

You should know that three independent parameters for generalized triangle (ASA or SSS) are needed to prove uniqueness.

if I have the point that the diagonals meet each other - Can I know the quadrilateral that have them?