MATLAB: System of 3 equation with 3 unknown, with two 2nd order equation

system solve unknown

Hi,
I am trying to solve a system of 3 equation with 3 unknown, with two 2nd order equation using this code, but unfortunately I don't understand why I do not obtain the result. Thank you in advance
syms QV3 QV2 QVl
eqn1 = 17.79  == QV3 + QV2+QVl;
eqn2 = 0.25*QV3.^2 == 0.14 * QV2^2;
eqn3 = 0.25*QV3.^2 == 3.89* QVl^2;
sol = solve([eqn1, eqn2, eqn3], [QV3 , QV2, QVl])
QV3Sol = sol.QV3
QV2Sol = sol.QV2
QVLsol = sol.QVl

Best Answer

The resuillts are strictly numerical (not containing symbolic variables), so use vpasolve instead of solve: 
syms QV3 QV2 QVl
eqn1 = 17.79  == QV3 + QV2+QVl;
eqn2 = 0.25*QV3.^2 == 0.14 * QV2^2;
eqn3 = 0.25*QV3.^2 == 3.89* QVl^2;
sol = vpasolve([eqn1, eqn2, eqn3], [QV3 , QV2, QVl])
QV3Sol = sol.QV3
QV2Sol = sol.QV2
QVLsol = sol.QVl
producing: 
QV3Sol =
 -214.86507282291207239983881906658
  6.8692131509642582629301134641865
 -30.161934858502498504240076448371
  8.5414023987755051080751389638705
 
QV2Sol =
  287.1255310312749278205504662485
 9.1793721884393279626540627283888
 40.305580843825110030042867255874
 11.413929063852511018159550534661
 
QVLsol =
 -54.470458208362855420711647181924
  1.7414146605964137744158238074247
  7.6463540146773884741972091924965
 -2.1653314626280161262346894985318
These are however still symbolic values. To use them in other calculations, convert them to double-precision values with the double function.
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