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 ArcTan

 http://functions.wolfram.com/01.15.04.0002.01

 Input Form

 ArcTan[-x, -y] == ArcTan[x, y] /; Element[y, Reals] && Inequality[-(Pi/2), Less, Arg[x], LessEqual, Pi/2]

 Standard Form

 Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["ArcTan", "[", RowBox[List[RowBox[List["-", "x"]], ",", RowBox[List["-", "y"]]]], "]"]], "\[Equal]", RowBox[List["ArcTan", "[", RowBox[List["x", ",", "y"]], "]"]]]], "/;", RowBox[List[RowBox[List["y", "\[Element]", "Reals"]], "\[And]", RowBox[List[RowBox[List["-", FractionBox["\[Pi]", "2"]]], "<", RowBox[List["Arg", "[", "x", "]"]], "\[LessEqual]", FractionBox["\[Pi]", "2"]]]]]]]]]

 MathML Form

 tan - 1 ( - x , - y ) tan - 1 ( x , y ) /; y TagBox["\[DoubleStruckCapitalR]", Function[Reals]] - π 2 < arg ( x ) π 2 Condition -1 x -1 y x y y Inequality -1 2 -1 x 2 -1 [/itex]

 Rule Form

 Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["ArcTan", "[", RowBox[List[RowBox[List["-", "x_"]], ",", RowBox[List["-", "y_"]]]], "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List["ArcTan", "[", RowBox[List["x", ",", "y"]], "]"]], "/;", RowBox[List[RowBox[List["y", "\[Element]", "Reals"]], "&&", RowBox[List[RowBox[List["-", FractionBox["\[Pi]", "2"]]], "<", RowBox[List["Arg", "[", "x", "]"]], "\[LessEqual]", FractionBox["\[Pi]", "2"]]]]]]]]]]]

 Date Added to functions.wolfram.com (modification date)

 2001-10-29