solution for symbolic equation: matlab can't find a solution for my equation

Question

Antonio Benigni 2021-11-27

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1597154-solution-for-symbolic-equation-matlab-can-t-find-a-solution-for-my-equation

评论： Walter Roberson 2021-11-30

i have this equation

J(x) =(4*C*Mo*mom*mom1*mus*sin(o)*(C^2 + 3*x^2))/(l*pi*(C^2 - x^2)^3) - (12*C*Mo*mom*mom1*x*cos(o)*(C^2 - x^2)^3*(C^2 + x^2))/(pi*(C + x)^7*(C - x)^7)==0

x is the only variable and the other parameters are positive real numbers

i try to find a solution for x based on the others parameter (there are many solutions, i found it on paper), but using the command solve i get this

root(3*mus*z^4*sin(o) + 3*l*z^3*cos(o) - 2*C^2*mus*z^2*sin(o) + 3*C^2*l*z*cos(o) - C^4*mus*sin(o), z)

this is the same solution i found on paper and thats how i know there are at least 2 real solutions but the program doesn't go on and don't give me the actual family of solution

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Paul 2021-11-27

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1597154-solution-for-symbolic-equation-matlab-can-t-find-a-solution-for-my-equation#answer_841594

Use the ReturnConditions option to see all of the solutions

syms x C Mo mom mom1 mus o l

J(x) =(4*C*Mo*mom*mom1*mus*sin(o)*(C^2 + 3*x^2))/(l*sym(pi)*(C^2 - x^2)^3) - (12*C*Mo*mom*mom1*x*cos(o)*(C^2 - x^2)^3*(C^2 + x^2))/(sym(pi)*(C + x)^7*(C - x)^7)==0;

sol = solve(J(x),x,'ReturnConditions',true)

sol = struct with fields:

x: [4×1 sym] parameters: [1×0 sym] conditions: [4×1 sym]

sol.x

ans =

However, specifying all the parameters as real and positive yields a much different result

syms x C Mo mom mom1 mus o l positive
J(x) =(4*C*Mo*mom*mom1*mus*sin(o)*(C^2 + 3*x^2))/(l*sym(pi)*(C^2 - x^2)^3) - (12*C*Mo*mom*mom1*x*cos(o)*(C^2 - x^2)^3*(C^2 + x^2))/(sym(pi)*(C + x)^7*(C - x)^7)==0;
sol = solve(J(x),x,'ReturnConditions',true)
sol = struct with fields:
             x: z1
    parameters: z1
    conditions: C ~= z1 & mus*sin(o)*C^4 + 2*mus*sin(o)*C^2*z1^2 == 3*l*cos(o)*C^2*z1 + 3*mus*sin(o)*z1^4 + 3*l*cos(o)*z1^3 & ~in(o/pi, 'integer') & 0 < z1

Ignoring analytic constraints leads to something a bit simpler

syms x C Mo mom mom1 mus o l positive
J(x) =(4*C*Mo*mom*mom1*mus*sin(o)*(C^2 + 3*x^2))/(l*sym(pi)*(C^2 - x^2)^3) - (12*C*Mo*mom*mom1*x*cos(o)*(C^2 - x^2)^3*(C^2 + x^2))/(sym(pi)*(C + x)^7*(C - x)^7)==0;
sol = solve(J(x),x,'ReturnConditions',true,'IgnoreAnalyticConstraints',true)
sol = struct with fields:
             x: z1
    parameters: z1
    conditions: 0 < z1 & mus*sin(o)*C^4 + 2*mus*sin(o)*C^2*z1^2 - 3*l*cos(o)*C^2*z1 - 3*mus*sin(o)*z1^4 - 3*l*cos(o)*z1^3 == 0