Three charges +q,+q and -q are placed on the circumference of a circle of radius r .The force on charge +Q placed at the centre of the circle will be 

Dear student,
Your question is not clear and appears to be incomplete. Recheck your question and please provide us with the exact figure associated with the question as the position of the three charges +q, +q, and -q on the circumference is not clear and and without knowing their exact position we can't solve it by assuming as different assumptions about the positions will lead to a different result. So please give us the complete question including the figure so that we can provide you with some meaningful help.
Look forward to hearing from you again!
Regards.

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