MATLAB: Solve issue in Matlab R2018b

MATLABsolveSymbolic Math Toolbox

I was using Matlab R2014a earlier where I was able to solve this equation easily, but in Matlab R2018b it is returning the
error in sol = solve(eqn,a,[0 pi]); I couldn't figure out why. The working code in Matlab 2014a is
syms a T
v2=-2.3750
g=1;
b=0;
e2=0.5;
k=2.5;
w=-2*cos(k);
eqn = sin(3*k+a)./sin(2*k+a)==v2-w+(g.*T.^2)+(e2.*T.^2.*sin(k)^2)./(sin(2*k+a)^2+b*T.^2*sin(k).^2);
sol = solve(eqn,a,[0 pi]);
digits(5)
solutions = vpa(subs(sol),3)
Please note that "a" is to be bound to take values between 0 and pi.

Best Answer

Restrict ‘a’ using an assume call.
Try this: 
syms a T
assume(a >= 0 & a <= pi)
v2=-2.3750
g=1;
b=0;
e2=0.5;
k=2.5;
w=-2*cos(k);
eqn = sin(3*k+a)./sin(2*k+a)==v2-w+(g.*T.^2)+(e2.*T.^2.*sin(k)^2)./(sin(2*k+a)^2+b*T.^2*sin(k).^2);
[sol,prms,conds] = solve(eqn,a, 'ReturnConditions',true)
digits(5)
solutions = vpa(subs(sol),3)
vpaconds = vpa(conds)
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