Math.Atan2 Method (Double, Double)
Note 

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Returns the angle whose tangent is the quotient of two specified numbers.
Assembly: mscorlib (in mscorlib.dll)
Parameters
 y

Type:
System.Double
The y coordinate of a point.
 x

Type:
System.Double
The x coordinate of a point.
Return Value
Type: System.DoubleAn angle, θ, measured in radians, such that π≤θ≤π, and tan(θ) = y / x, where (x, y) is a point in the Cartesian plane. Observe the following:
For (x, y) in quadrant 1, 0 < θ < π/2.
For (x, y) in quadrant 2, π/2 < θ≤π.
For (x, y) in quadrant 3, π < θ < π/2.
For (x, y) in quadrant 4, π/2 < θ < 0.
For points on the boundaries of the quadrants, the return value is the following:
If y is 0 and x is not negative, θ = 0.
If y is 0 and x is negative, θ = π.
If y is positive and x is 0, θ = π/2.
If y is negative and x is 0, θ = π/2.
If y is 0 and x is 0, θ = 0.
If x or y is NaN, or if x and y are either PositiveInfinity or NegativeInfinity, the method returns NaN.
The return value is the angle in the Cartesian plane formed by the xaxis, and a vector starting from the origin, (0,0), and terminating at the point, (x,y).
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1