# 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 QVleqn1 = 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.QV3QV2Sol = sol.QV2QVLsol = sol.QVl

#### Best Answer

• The resuillts are strictly numerical (not containing symbolic variables), so use vpasolve instead of solve:
syms QV3 QV2 QVleqn1 = 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.QV3QV2Sol = sol.QV2QVLsol = 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.