设,顶点为O,三个截点为A,B,C则,△OAB,OBC,OCA均为直角△OA^2+OB^2=AB^2,OB^2+OC^2=BC^2,OC^2+OA^2=CA^2则,AB^2+BC^2>CA^2而,CA^2=AB^2+BC^2-2AB*BCcosABC则,AB^2+BC^2>AB^2+BC^2-2AB*BCcosABC则cosABC>0,角ABC为锐角...