Solve algebraic equation: define for which variable to solve

Question

Jurrien Plijter 2020-5-20

0
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回答： Jurrien Plijter 2020-5-20

Hi,

I try to solve a simple algebraic equation, defined by finding the variables for which the partial derivates of 'eqn1' are all 0. I want to solve the equation for the variables x, y and lambda. Such that those three variables are expressed as a function of the other variables. In this case, i look for the solution that:

x = 0;
lambda = - m*g / 2 / y; 
y= + or - R 

How can i define such an output? Thank you for your help! (This is a simplified problem of a more complicated case.)

syms x y m g R lambda L
eqn1 = L == m*g*(y+R) + lambda * (x^2 + y^2 - R^2); 
dldx1       =diff(eqn1,x);
dldx2       =diff(eqn1,y);
dldlamba    =diff(eqn1, lambda);
dl=[dldx1 ; dldx2 ; dldlamba]
[solx, soly, sollambda]=solve(dldx1==0, dldx2==0, dldlamba==0)