Simplify[
      ArcCos[(25/(Sqrt[34 29]))] + ArcCos[(9/(Sqrt[34  13]))] + 
        ArcCos[(4/(Sqrt[13 29]))]]

FullSimplify[
      ArcCos[(25/(Sqrt[34 29]))] + ArcCos[(9/(Sqrt[34  13]))] + 
        ArcCos[(4/(Sqrt[13 29]))]]

N[ArcCos[(25/(Sqrt[34 29]))] + ArcCos[(9/(Sqrt[34  13]))] + 
        ArcCos[(4/(Sqrt[13 29]))]]

N[ArcCos[(25/(Sqrt[34 29]))] + ArcCos[(9/(Sqrt[34  13]))] + 
        ArcCos[(4/(Sqrt[13 29]))], 50]

N[Pi - ( 
          ArcCos[(25/(Sqrt[34 29]))] + ArcCos[(9/(Sqrt[34  13]))] + 
            ArcCos[(4/(Sqrt[13 29]))])]