Torque on a rectangular coil in a magnetic field

[Sanosuke Sagara]

This is another question that I have think for very long hours but still find the solution to this question.I have my question in the attachment that followed.

 

[learningphysics]

No. Not that way. You can calculate r using purely geometry.

 

[Sanosuke Sagara]

Thanks for your help and I finally get the answer to my question.