ArcGIS Pro 2.6 API Reference Guide
SemiMajorAxis Property
Example 

ArcGIS.Core.Geometry Namespace > EllipticArcSegment Class : SemiMajorAxis Property
Gets the semi-major axis. The semi-major axis is the larger of the two axes, and the semi-minor axis is the smaller axis. The semi-major axis always lies on the line between 0 and PI radians.
Syntax
public double SemiMajorAxis {get;}
Public ReadOnly Property SemiMajorAxis As Double

Property Value

A double representing the semi-major axis.
Example
// retrieve the curve's control points
EllipticArcSegment arc = EllipticArcBuilder.CreateEllipticArcSegment(arcSegment);
MapPoint startPt = arc.StartPoint;
MapPoint endPt = arc.EndPoint;
Coordinate2D centerPt = arc.CenterPoint;
bool isCircular = arc.IsCircular;
bool isMinor = arc.IsMinor;
bool isCounterClockwise = arc.IsCounterClockwise;
bool isCurve = arc.IsCurve;
double len = arc.Length;
double ratio = arc.MinorMajorRatio;

double semiMajorAxis, semiMinorAxis;
// get the axes
arc.GetAxes(out semiMajorAxis, out semiMinorAxis);
// or use the properties
// semiMajorAxis = arc.SemiMajorAxis;
// semiMinorAxis = arc.SemiMinorAxis;

double startAngle, centralAngle, rotationAngle;
// or use QueryCoords to get complete information
arc.QueryCoords(out centerPt, out startAngle, out centralAngle, out rotationAngle, out semiMajorAxis, out semiMinorAxis);

// use properties to get angle information
//double endAngle = arc.EndAngle;
//centralAngle = arc.CentralAngle;
//rotationAngle = arc.RotationAngle;
//startAngle = arc.StartAngle;
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

EllipticArcSegment Class
EllipticArcSegment Members