Question: solving a system of trigonometric equations

Hey guys I need help solving the following system of trigonometric equations

invEqs := [a[3]*cos(theta[1] + theta[2] + theta[3]) + a[2]*cos(theta[1] + theta[2]) + a[1]*cos(theta[1]) = px, a[3]*sin(theta[1] + theta[2] + theta[3]) + a[2]*sin(theta[1] + theta[2]) + a[1]*sin(theta[1]) = py, theta[1] + theta[2] + theta[3] = phi]

which everything except theta[1], theta[2], theta[3] are known.


here what I came up with but no success

res := solve(invEqs, {theta[2], theta[3], sin(theta[1])})

or better be like this but no output printed

res := solve(invEqs, {theta[1], theta[3], cos(theta[2])})

thanks in advance

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