error using solve for equation with one unknown

Question

Andrei Costache 2014-12-8

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评论： Andrei Costache 2014-12-8

Hi!

I am triyng to solve for 'p' with the code:

g = linspace(0*(pi/180),180*(pi/180),10);

a = 0.7644;

b = 0.2101;

cof = 0.2;

eq = solve('(1+g)*cos(p*pi)*(sin(p*g)^2-p^2*sin(g)^2)+(1/2)*(1-a)*sin(p*pi)*(sin(2*p*g)+p*sin(2*g))+cof*sin(p*pi)*((1-a)*p*(1+p)*sin(g)^2-2*b*(sin(p*g)^2-p^2*sin(g)^2))','p');

u = subs(eq,g);

and I get the following error:

Warning: Cannot find explicit solution. > In solve at 319 In stress_int at 56 Error using symengine (line 58) Number of elements in NEW must match number in OLD.

Error in sym/subs>mupadsubs (line 139) G = mupadmex('symobj::fullsubs',F.s,X2,Y2);

Error in sym/subs (line 124) G = mupadsubs(F,X,Y);

Error in stress_int (line 57) u = subs(eq,g);

Any ideas why? Please help!

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Answer 1

Zoltán Csáti 2014-12-8

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The values of a, b, cof are not retained in the string, so use

syms p
solve((1+g)*cos(p*pi)*(sin(p*g)^2-p^2*sin(g)^2)+(1/2)*(1-a)*sin(p*pi)*(sin(
2*p*g)+p*sin(2*g))+cof*sin(p*pi)*((1-a)*p*(1+p)*sin(g)^2-2*b*(sin(p*g)^2-p^2*sin(g)^2)),p);

instead. But g is a vector so you will get an overdetermined system.

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Zoltán Csáti 2014-12-8

It returned the same warning for me too. It means, that the Symbolic Math Toolbox could not solve it for p. I tried it with Maple and that couldn't solve it either. So try to solve it numerically, you can't do other things.

Andrei Costache 2014-12-8