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Integrate[1/(a*Sqrt[c^2 - d^2*x^2]), x] ==
ArcTan[(d*x)/Sqrt[c^2 - d^2*x^2]]/(a*d)

                  1
Integrate[------------------, x] == 
                  2    2  2
          a Sqrt[c  - d  x ]

             d x
ArcTan[----------------]
             2    2  2
       Sqrt[c  - d  x ]
------------------------
          a d