Stuck in a (easy?) formula

[Efrain R Portales <efrain12@xxxxxxxxxxxx>]

Hi guys,
I need some help from someone fresh with simple math and geometry.
What I'm trying to find is a formula that tells me when today's close,
yesterday's close and the day before yesterday's close form a triangle
greater or smaller than 90 degrees.
Let's call :
C = today's close
C1 = yesterday close
C2  = the close two days ago

assume :
1).- C2 >C1
2). - C2> C
3). - C1< C

If you join those three points, you'll have a triangle with the internal
angle at vertex C1 being the
only one that can be greater or smaller than 90 degrees under the assumptions
listed above.
In short, I'm trying to find a formula using the difference between
today's Close and yesterday's
Close to know whether the angle at C1 is greater or smaller than 90
degrees
What confuses me is, while constructing the triangle, the horizontal
distance ( the time axis ) between consecutive prices is one unit, be one
day or one week or whatever, but it's one unit.
 
This is the approximate relative position of Close prices :
 

                                               
C2
 
 
                                                                             
C
 
                                                              
C1

I looks like a simple formula involving the vertical distances ( C2
- C1 ), ( C2 - C ), ( C - C1 ) and
two time units. I just can't write it. Maybe I'm a little rusty ( got
out of college a looong time ago !).
Any help will be greatly appreciated !.

Have a good one,

Efrain R Portales
efrain12@xxxxxxxxxxxx