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   http://functions.wolfram.com/01.18.16.0006.01
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    ArcSec[x] + ArcSec[y] == 
  Sign[(x + y)/(x y)] ArcCos[1/(x y) - Sqrt[1 - 1/x^2] Sqrt[1 - 1/y^2]] + 
   Pi (1 - Sign[(x + y)/(x y)]) /; Abs[x] > 1 || Abs[y] > 1 || x y > 0 
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   Cell[BoxData[RowBox[List[RowBox[List[RowBox[List[RowBox[List["ArcSec", "[", "x", "]"]], "+", RowBox[List["ArcSec", "[", "y", "]"]]]], "\[Equal]", RowBox[List[RowBox[List[RowBox[List["Sign", "[", FractionBox[RowBox[List["x", "+", "y"]], RowBox[List["x", " ", "y"]]], "]"]], " ", RowBox[List["ArcCos", "[", RowBox[List[FractionBox["1", RowBox[List["x", " ", "y"]]], "-", RowBox[List[SqrtBox[RowBox[List["1", "-", FractionBox["1", SuperscriptBox["x", "2"]]]]], " ", SqrtBox[RowBox[List["1", "-", FractionBox["1", SuperscriptBox["y", "2"]]]]]]]]], "]"]]]], "+", RowBox[List["\[Pi]", " ", RowBox[List["(", RowBox[List["1", "-", RowBox[List["Sign", "[", FractionBox[RowBox[List["x", "+", "y"]], RowBox[List["x", " ", "y"]]], "]"]]]], ")"]]]]]]]], "/;", RowBox[List[RowBox[List[RowBox[List["Abs", "[", "x", "]"]], ">", "1"]], "||", RowBox[List[RowBox[List["Abs", "[", "y", "]"]], ">", "1"]], "||", RowBox[List[RowBox[List["x", " ", "y"]], ">", "0"]]]]]]]] 
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   <math xmlns='http://www.w3.org/1998/Math/MathML' mathematica:form='TraditionalForm' xmlns:mathematica='http://www.wolfram.com/XML/'>  <semantics>  <mrow>  <mrow>  <mrow>  <mrow>  <msup>  <mi> sec </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mi> x </mi>  <mo> ) </mo>  </mrow>  <mo> + </mo>  <mrow>  <msup>  <mi> sec </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mi> y </mi>  <mo> ) </mo>  </mrow>  </mrow>  <mo> ⩵ </mo>  <mrow>  <mrow>  <mi> π </mi>  <mo> ⁢ </mo>  <mrow>  <mo> ( </mo>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mrow>  <mi> sgn </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mfrac>  <mrow>  <mi> x </mi>  <mo> + </mo>  <mi> y </mi>  </mrow>  <mrow>  <mi> x </mi>  <mo> ⁢ </mo>  <mi> y </mi>  </mrow>  </mfrac>  <mo> ) </mo>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  </mrow>  <mo> + </mo>  <mrow>  <mrow>  <msup>  <mi> cos </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mrow>  <mfrac>  <mn> 1 </mn>  <mrow>  <mi> x </mi>  <mo> ⁢ </mo>  <mi> y </mi>  </mrow>  </mfrac>  <mo> - </mo>  <mrow>  <msqrt>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <msup>  <mi> x </mi>  <mn> 2 </mn>  </msup>  </mfrac>  </mrow>  </msqrt>  <mo> ⁢ </mo>  <msqrt>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <msup>  <mi> y </mi>  <mn> 2 </mn>  </msup>  </mfrac>  </mrow>  </msqrt>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mi> sgn </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mfrac>  <mrow>  <mi> x </mi>  <mo> + </mo>  <mi> y </mi>  </mrow>  <mrow>  <mi> x </mi>  <mo> ⁢ </mo>  <mi> y </mi>  </mrow>  </mfrac>  <mo> ) </mo>  </mrow>  </mrow>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mrow>  <mrow>  <semantics>  <mo> ❘ </mo>  <annotation encoding='Mathematica'> "\[LeftBracketingBar]" </annotation>  </semantics>  <mi> x </mi>  <semantics>  <mo> ❘ </mo>  <annotation encoding='Mathematica'> "\[RightBracketingBar]" </annotation>  </semantics>  </mrow>  <mo> > </mo>  <mn> 1 </mn>  </mrow>  <mo> ∨ </mo>  <mrow>  <mrow>  <semantics>  <mo> ❘ </mo>  <annotation encoding='Mathematica'> "\[LeftBracketingBar]" </annotation>  </semantics>  <mi> y </mi>  <semantics>  <mo> ❘ </mo>  <annotation encoding='Mathematica'> "\[RightBracketingBar]" </annotation>  </semantics>  </mrow>  <mo> > </mo>  <mn> 1 </mn>  </mrow>  <mo> ∨ </mo>  <mrow>  <mrow>  <mi> x </mi>  <mo> ⁢ </mo>  <mi> y </mi>  </mrow>  <mo> > </mo>  <mn> 0 </mn>  </mrow>  </mrow>  </mrow>  <annotation-xml encoding='MathML-Content'>  <apply>  <ci> Condition </ci>  <apply>  <eq />  <apply>  <plus />  <apply>  <arcsec />  <ci> x </ci>  </apply>  <apply>  <arcsec />  <ci> y </ci>  </apply>  </apply>  <apply>  <plus />  <apply>  <times />  <pi />  <apply>  <plus />  <cn type='integer'> 1 </cn>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <ci> Sign </ci>  <apply>  <times />  <apply>  <plus />  <ci> x </ci>  <ci> y </ci>  </apply>  <apply>  <power />  <apply>  <times />  <ci> x </ci>  <ci> y </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <times />  <apply>  <arccos />  <apply>  <plus />  <apply>  <times />  <cn type='integer'> 1 </cn>  <apply>  <power />  <apply>  <times />  <ci> x </ci>  <ci> y </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <times />  <apply>  <power />  <apply>  <plus />  <cn type='integer'> 1 </cn>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <times />  <cn type='integer'> 1 </cn>  <apply>  <power />  <apply>  <power />  <ci> x </ci>  <cn type='integer'> 2 </cn>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </apply>  <cn type='rational'> 1 <sep /> 2 </cn>  </apply>  <apply>  <power />  <apply>  <plus />  <cn type='integer'> 1 </cn>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <times />  <cn type='integer'> 1 </cn>  <apply>  <power />  <apply>  <power />  <ci> y </ci>  <cn type='integer'> 2 </cn>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </apply>  <cn type='rational'> 1 <sep /> 2 </cn>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <ci> Sign </ci>  <apply>  <times />  <apply>  <plus />  <ci> x </ci>  <ci> y </ci>  </apply>  <apply>  <power />  <apply>  <times />  <ci> x </ci>  <ci> y </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <or />  <apply>  <gt />  <apply>  <abs />  <ci> x </ci>  </apply>  <cn type='integer'> 1 </cn>  </apply>  <apply>  <gt />  <apply>  <abs />  <ci> y </ci>  </apply>  <cn type='integer'> 1 </cn>  </apply>  <apply>  <gt />  <apply>  <times />  <ci> x </ci>  <ci> y </ci>  </apply>  <cn type='integer'> 0 </cn>  </apply>  </apply>  </apply>  </annotation-xml>  </semantics>  </math> 
   
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  | Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List[RowBox[List["ArcSec", "[", "x_", "]"]], "+", RowBox[List["ArcSec", "[", "y_", "]"]]]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[RowBox[List[RowBox[List["Sign", "[", FractionBox[RowBox[List["x", "+", "y"]], RowBox[List["x", " ", "y"]]], "]"]], " ", RowBox[List["ArcCos", "[", RowBox[List[FractionBox["1", RowBox[List["x", " ", "y"]]], "-", RowBox[List[SqrtBox[RowBox[List["1", "-", FractionBox["1", SuperscriptBox["x", "2"]]]]], " ", SqrtBox[RowBox[List["1", "-", FractionBox["1", SuperscriptBox["y", "2"]]]]]]]]], "]"]]]], "+", RowBox[List["\[Pi]", " ", RowBox[List["(", RowBox[List["1", "-", RowBox[List["Sign", "[", FractionBox[RowBox[List["x", "+", "y"]], RowBox[List["x", " ", "y"]]], "]"]]]], ")"]]]]]], "/;", RowBox[List[RowBox[List[RowBox[List["Abs", "[", "x", "]"]], ">", "1"]], "||", RowBox[List[RowBox[List["Abs", "[", "y", "]"]], ">", "1"]], "||", RowBox[List[RowBox[List["x", " ", "y"]], ">", "0"]]]]]]]]]]  |  
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   Date Added to functions.wolfram.com (modification date)
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