solving implicit equation with 3 variables knowing 2 of them

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Hi, I'm trying to solve the the equation shown in the attachment as you see it's function of three variables M,theta, and beta where lamda is 1.4, so given M and theta I would like to get the two corresponding values of beta. I have tried this:
syms beta;
M1= 5;
theta=20*pi/180;
gama=1.4;
eqn=tan(theta)==2*cot(beta)*((M1^2*sin(beta)^2-1)/M1^2*(gama+cos(2*beta)+2));
beta=solve(eqn,beta,'ReturnConditions',true);
% betadeg=beta*180/pi;
vpa(beta)
But I keep getting these errors
Error using sym>tomupad (line 1256)
Unable to convert 'struct' to 'sym'.
Error in sym (line 199)
S.s = tomupad(x);
Error in vpa (line 46)
ss = sym(s);
Error in untitled (line 8)
vpa(beta)
I would really appreciate your help. Thanks.

采纳的回答

Birdman
Birdman 2018-3-28
Replace
vpa(beta)
with
vpa(beta.beta,3)
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