# [✓] Solve systems of equations with trigonometric expressions?

Posted 1 year ago
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 I am fairly new to Mathematica, so this may be a simplistic question, but I want to learn. I know how to use the Solve function to solve a single equation like this: Solve[Cot[c Degree] == 5.792, c] and I know how to use Solve to solve a system of multiple equations like this: Solve[{5*j + 2*r == 5, 6*j - 3*r == 9}, {j, r}] However, when I try to solve a system of multiple equations that involves trigonometric expressions like this (some variables previously defined): Solve[{Fr == Cos[phi Degree]*Fb + Sin[theta Degree]*Fa, 0 == Cos[theta Degree]*Fa - Sin[phi Degree]*Fb}, {Fa, theta}] // N I get only a partially correct answer along with some results that are difficult to understand: {{Fa -> -883.103, theta -> ConditionalExpression[57.2958 (-2.10618 + 6.28319 C[1]), C[1] \[Element] Integers]}, {Fa -> 883.103, theta -> ConditionalExpression[57.2958 (1.03541 + 6.28319 C[1]), C[1] \[Element] Integers]}} What do I need to do to get Mathematica to give me a value of theta in degrees? I am sure there is a straightforward solution, but I just can't seem to find it. Thank you!
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Posted 1 year ago
 A ConditionalExpression is valid only if the given condition is satisfied. In this case, C[1] must be an integer coefficient. The solution don't seem problematic to me: eqs = {Fr == Cos[phi Degree] Fb + Sin[theta Degree] Fa, 0 == Cos[theta Degree] Fa - Sin[phi Degree] Fb}; sol = Simplify@Solve[eqs, {Fa, theta}]; Simplify[eqs /. sol, Element[C[1], Integers]] You get single solutions by giving integer numerical values to C[1]: Simplify[sol /. C[1] -> 3] 
 Although I don't really understand what you want to do I think there is another possible approach. For the time being I forget the Degrees, any angles may be easily converted to degrees later on.Your second equation is easily solved for Sin [ phi ] In[1]:= Fr1 = Fb Cos[phi] + Fa Sin[theta]; lsg = Solve[0 == Fa Cos[theta] - Fb Sin[phi], Sin[phi]] // Flatten Out[2]= {Sin[phi] -> (Fa Cos[theta])/Fb} In your first equation convert Cos [ phi ] to a Sin-expression and and put the solution for Sin [ phi ] in In[3]:= Fr2 = Fr1 /. Cos[phi] -> Sqrt[1 - Sin[phi]^2] Out[3]= Fb Sqrt[1 - Sin[phi]^2] + Fa Sin[theta] In[4]:= Fr3 = Fr2 /. lsg Out[4]= Fb Sqrt[1 - (Fa^2 Cos[theta]^2)/Fb^2] + Fa Sin[theta] Simplify this a bit In[5]:= Fr4 = Simplify[Fr3 /. a_ Sqrt[x_] -> Sqrt[a^2 x]] Out[5]= Sqrt[Fb^2 - Fa^2 Cos[theta]^2] + Fa Sin[theta] and solve for theta In[6]:= Solve[Fr4 == Fr, theta] During evaluation of In[6]:= Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >> Out[6]= { {theta -> -ArcCos[-( Sqrt[-Fa^4 + 2 Fa^2 Fb^2 - Fb^4 + 2 Fa^2 Fr^2 + 2 Fb^2 Fr^2 - Fr^4]/(2 Fa Fr))]}, {theta -> ArcCos[-(Sqrt[-Fa^4 + 2 Fa^2 Fb^2 - Fb^4 + 2 Fa^2 Fr^2 + 2 Fb^2 Fr^2 - Fr^4]/(2 Fa Fr))]}, {theta -> -ArcCos[ Sqrt[-Fa^4 + 2 Fa^2 Fb^2 - Fb^4 + 2 Fa^2 Fr^2 + 2 Fb^2 Fr^2 - Fr^4]/(2 Fa Fr)]}, {theta -> ArcCos[Sqrt[-Fa^4 + 2 Fa^2 Fb^2 - Fb^4 + 2 Fa^2 Fr^2 + 2 Fb^2 Fr^2 - Fr^4]/(2 Fa Fr)]}} To get real solutions you should consider the expression under the square root, which in this case must be > 0 In[10]:= Factor[-Fa^4 + 2 Fa^2 Fb^2 - Fb^4 + 2 Fa^2 Fr^2 + 2 Fb^2 Fr^2 - Fr^4] Out[10]= -(Fa - Fb - Fr) (Fa + Fb - Fr) (Fa - Fb + Fr) (Fa + Fb + Fr) Showing that one or three factors must be negative.