The net force acting on test charge is resultant of two force i.e.,  √2 F
The magnitude of force on test charge due to one charge Q is:
        F = k qQ / r²
where k = 9 × 10^9
Similarly the force due to second charge will be:
     F = k qQ / r²
Now both the charges of magnitude Q is present and it is pointed at right angle to each other then the force  will be resultant i.e.,
          F = √ F² + F² = F√2
 Hence, net force acting on test charge is √2 F.
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