Symbolize but the solutions number of the equation changed

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Lingling Fan 2019-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/445386-symbolize-but-the-solutions-number-of-the-equation-changed

评论： chirag rohit 2019-3-16

Screen Shot 2019-02-16 at 11.57.29 PM.png

To solve the det A ==0 equation, if I symbolize eta_yy, eta_xy..., I get q_sol as 4*1. But if I take eta_yx, eta_xy... as a number, I get q_sol as 6*1. Why is it?

Thank you in advance

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Answer 1

chirag rohit 2019-3-16

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https://ww2.mathworks.cn/matlabcentral/answers/445386-symbolize-but-the-solutions-number-of-the-equation-changed#answer_365659

clc

clear all;

syms q

eta_yy = 2;

eta_yx = 3;

eta_xy = 4;

eta_xx = 2;

eta_zy = 4;

eta_zx = 5;

eta_yz = 6;

eta_xz = 3;

eta_zz = 5;

ky = 8;

kx = 7;

k0 = 2.5;

A2 = [eta_yy, -eta_yx; -eta_xy, eta_xx];

A1a = [-ky*eta_zy, ky*eta_zx; kx*eta_zy, -kx*eta_zx];

A1b = [-ky*eta_yz, ky*eta_yz; kx*eta_xz, -kx*eta_xz];

A1 = A1a + A1b;

A0a = [ky^2 * eta_zz, -kx * ky * eta_zz; -kx * ky * eta_zz, kx^2 * eta_zz];

A0b = [kx^2 * eta_yy - kx * ky * eta_yx, kx*ky*eta_yy - ky^2*eta_yx; ...

kx * ky * eta_xx - kx^2 * eta_xy, ky^2 * eta_xx - kx * ky * eta_xy];

A0 = A0a + A0b - k0^2*eye(2);

Am1 = [ky^2 * kx * eta_zx - kx^2 * ky * eta_zy, ky^3 * eta_zx - ky^2 * kx *eta_zy;...

kx^3 * eta_zy - kx^2 * ky * eta_zx, kx^2 * ky * eta_zy - ky^2 * kx * eta_zx];

A_mat = A2 * q^2 + A1 * q + A0 + Am1 * q^(-1);

% simplify(A_mat);

collect(det(A_mat),q);

q_sol = solve(det(A_mat)==0,q)

>> vpa(q_sol,4)

ans =

0.2184 - 10.71i

0.2184 + 10.71i

2.867

25.07

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chirag rohit 2019-3-16

Respected ma'am,

Here i have taken random values of eta_xx,.... And tried to solve eqation. I get q_sol as 4*1. In both ways i get q_sol as 4*1. If you are choosing some specified values for variables please mention.

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