Solving a quartic with matlab

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Joshua
Joshua 2014-3-2
回答： Paul 2014-3-2
Hi, I'm new to matlab and im struggling to solve a quartic equation with 4 variables, so far ive got this:
clear
g=9.81;
h=10;
u=1;
d=0.5;
c=0.5;
a=u./sqrt(g.*h)
syms a b c d
solve(((a^(2)*b^4)/2)+2*c*a^(2)*b^(3)-(2-2*d+a^(2))*b^(2)-(4*c+2*c*a^(2)-4)*b+((a^(2)/2)+4*c-2*c^(2)*d-2),b)
which gives me the answer:
a =
0.1010
ans =
RootOf(a^2*z^4 + 4*a^2*c*z^3 - 2*a^2*z^2 + 4*d*z^2 - 4*z^2 - 4*a^2*c*z - 8*c*z + 8*z - 4*c^2*d + 8*c + a^2 - 4, z)
the value for a is what I expected, however I expected 4 real numbers for my value for b. Can you offer any help interpreting this answer?
Thanks Joshua

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回答（1 个）

Paul
Paul 2014-3-2
Matlab will solve up to a 3rd order polynomial by default. To get an explicit result you have to specify a max degree, in your case 4. See: http://www.mathworks.nl/help/symbolic/solve.html
So:
solve(((a^(2)*b^4)/2)+2*c*a^(2)*b^(3)-(2-2*d+a^(2))*b^(2)-(4*c+2*c*a^(2)-4)*b+((a^(2)/2)+4*c-2*c^(2)*d-2),b, 'MaxDegree', 4)

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