|  1 |    initial version    |  
Using the keyword argument algorithm='sympy' leads to more success:
sage: solve(cos(theta)==sin(theta), theta, algorithm='sympy')
[ImageSet(Lambda(_n, 2*_n*pi + 5*pi/4), Integers),
 ImageSet(Lambda(_n, 2*_n*pi + pi/4), Integers)]
 
 
                
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