Function "solve" returns a struct

Hi, I have the following equation:

where are defined by syms.

I need to find in that equation, so my solution should be:

I have used the following code:

sol=solve(equation==0, cos(q1))

However, what I get in my console is:

sol =

struct with fields:

q1: [0×1 sym]

q2: [0×1 sym]

Then I added:

sol.q1

and I got in the console:

ans =

Empty sym: 0-by-1

Answer 1

[Stephan]

syms a1 a2 q1 q2
eq = a1*cos(q1) + a2*cos(q1)*cos(q2) - a2*sin(q1)*sin(q2) == 0
sol = isolate(eq,cos(q1))
pretty(sol)