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MathGroup Archive 2002

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Mathematica returns empty set when I expect two different values

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32329] Mathematica returns empty set when I expect two different values
  • From: "spiraltooth" <spiraltooth at netzero.net>
  • Date: Tue, 15 Jan 2002 02:30:06 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

The following is a Mathematica file in which a spiral is described in 
polar coordinates. I am trying to find a point Ttheta,theta on the 
spiral given another point Tt,t . These two points are separated by the 
straight line distance Mtheta. Both Ttheta and Mtheta are functions of 
theta and are the unknowns. Thank
you for helping me.                                                      
                     Alan Notis


In(1)=     S=6Pi
Out(1)=   6Pi
In(2)=     MR=.7
Out(2)=   .7
In(3)=     t=Pi
Out(3)=   Pi
In(4)=     Itheta=MR+(theta / S)
Out(4)=   0.7+(theta / 6Pi)
In(5)=     It=MR+(t / S)
Out(5)=   0.8666667
In(6)=     CM=.025
Out(6)=   .025
In(7)=     CX=.275
Out(7)=   .275
In(8)=     Mtheta=CX-(((CX-CM)theta) / S)
Out(8)=   0.275-0.0132629theta
In(9)=     Ttheta=Itheta+Mtheta   (*  This describes a radius Ttheta 
given theta as a point on a spiral from ~-.1Pi to ~6.1Pi  *)
Out(9)=   0.975+0.0397887theta
In(10)=    Mt=CX-(((CX-CM)t) / S)
Out(10)=  0.233333
In(11)=    Tt=It+Mtheta   (*   This describes a radius Tt given t as 
a point on the same spiral as Ttheta,theta; Again t is demonstrated at 
Pi here but in reality will vary from 0 to 6Pi   *)
Out(11)=  1.14167-0.0132629theta
In(12)=     
alpha=ArcCos[((Ttheta^2)+(Tt^2)-(Mtheta^2))/(2(Ttheta)(Tt))]
Out(12)=  
ArcCos[(-(0.275-0.0132629theta)^2+(1.14167-0.0132629theta)^2+(0.975+0.0397887theta)^2) / 2(1.14167-0.0132629theta)(0.975+0.0397887theta)]
In(13)=    Solve[alpha==ArcCos[(Ttheta^2+Tt^2-Mtheta^2) / 
(2(Ttheta(Tt)))] , theta ]     (*   Here I'm Using the law of cosines to 
find theta where alpha is the angle between t and theta . I'm expecting 
two values for theta one where theta < t and one where theta > t . 
Mathematica seems unable to do this as it returns the null set.  *)
Out(13)=  {{}}
In(14)=    Solve[2X^2 + 14X +12 == 0 , X]     (*    But 
Mathematica of course has no problem solving an equation in a quadratic 
form. Do I need to load a certain package? Am I doing something 
mathematically wrong? Do I need to use ( and learn ) vector calculus?   
*)
Out(14)=  {{X -> -6},{X -> -1}}


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