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1/v + 1/u = 1/f
     m = -v/u = -3    =>     v = 3 u

hence,  1/(3u) + 1/u = 1/f
             4/(3u) = 1/f  =>  u =  4 f /3  = 1.3333 f

 The object is placed between  f and 2f  ie.,  focal point F and center of curvature C.

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