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I would like to be able to extract terms that contain either a symbol or a small expression from a larger one.

Example (with a somewhat complicated function to give the idea of what I am after): 

-R (δt + 2 θ2t Sin[θ1]) + 
 2 Cos[ϕ/
   2] (Sin[δ/
      2] (vz Sin[θ2] + 
       Cos[θ2] (vy Cos[θ3] + 
          h θ3t Sin[θ1] - vx Sin[θ3])) + 
    Cos[δ/
      2] (h (θ1t + θ3t Sin[θ2]) - 
       Sin[θ1] (vz Cos[θ2] - 
          vy Cos[θ3] Sin[θ2] + 
          vx Sin[θ2] Sin[θ3]))) - 
 2 l θ2t Cos[δ/2] Sin[θ1] Sin[ϕ/2] - 
 2 Cos[θ1] (-Cos[ϕ/2] (-h θ2t Sin[δ/2] + 
       Cos[δ/
         2] (vx Cos[θ3] + 
          vy Sin[θ3])) + θ3t Cos[θ2] (R + 
       l Cos[δ/2] Sin[ϕ/2]))

I would like Mma to return all the terms that multiply vx: 

2 Cos[ϕ/
  2] (Sin[δ/
    2]Sin[θ3]))-2 Cos[θ1] -Cos[ϕ/2] 
     Cos[δ/2] Cos[θ3]
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Coefficient[expr, vx] // Simplify

enter image description here

where expr is your expression.

Update

Coefficient[expr, Sin[\[Theta]2]] // Simplify

enter image description here

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2
  • $\begingroup$ Would this work if instead of vx I had an expression like $\sin\theta_2$? $\endgroup$ – usumdelphini Jan 29 '19 at 14:16
  • $\begingroup$ @usumdelphini Yes, I think so. Please see the updated part. $\endgroup$ – Αλέξανδρος Ζεγγ Jan 29 '19 at 14:30

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