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Topic: Heron's Formula HELP!!
Replies: 3   Last Post: Nov 16, 1996 1:02 PM

 Messages: [ Previous | Next ]
 James A Mccann Posts: 10 Registered: 12/6/04
Re: Heron's Formula HELP!!
Posted: Nov 16, 1996 12:57 PM

> Use a triangle with the sides 3, 4, and 5. You know that the angles
> are 30, 60, and 90 degrees. Use heron's formula and use another area
> of the triangle formula and there is your proof.

Sorry, but this is not a proof. If Heron's formula was not true and this
example disproved it, it would be a perfectly valid proof by
counterexample that it is not true. But Heron's formula is a generalized
statement-- it is a relationship between area and sides of ALL
triangles. Therefore a proof of it MUST prove its validity for all
triangles.It would have to use the definition of a triangle (a planar
polygon with three sides), perhaps some of the characteristics of all
triangles which may be be accepted because they were proven elsewhere
(such as the sum of internal angles being 180 degrees).

I cannot try to formulate a proof now because I don't remember Heron's
formula. However, I can suggest the following strategy which I think
might be useful for a proof by construction:

Start with another method of finding the area of a general triangle
(don't use the pythagorean theorem or trigonometry-- they require right
triangles). Use the following methods to convert angles into side
lengths (you may need to solve a system of simultanious equations):

For a triangle A B C with opposite angles a, b and c respectively,

Law of cosines:

A^2 = B^2 + C^2 - 2 A B cos( a )

Law of sines:

sin( a ) sin( b )
-------- = --------
A B

If I got one of these two laws wrong, somebody correct me. It's been a
while and I don't have a reference available.

-- Jim

===================================================
Jim McCann
Physics sophomore, Carnegie Mellon University
mccann+@andrew.cmu.edu
(412) 862 3003

Date Subject Author
11/16/96 James A Mccann
11/16/96 bob
11/16/96 James A Mccann