３点を通過する円弧の中心座標（ＶＢＡ）

平面上で３点を通過する円弧の中心座標を求めるプログラム例です。連立方程式の解法で行列演算を利用しているので、応用が利くと思います。

Sub テスト()

Dim A(1 To 2, 1 To 2) As Double
Dim B(1 To 2, 1 To 1) As Double
Dim I As Integer, X1 As Double, X2 As Double, X3 As Double
Dim Y1 As Double, Y2 As Double, Y3 As Double

With Worksheets("sheet11")
I = 1
Do
X1 = .Cells(I, 1)
X2 = .Cells(I + 1, 1)
X3 = .Cells(I + 2, 1)
Y1 = .Cells(I, 2)
Y2 = .Cells(I + 1, 2)
Y3 = .Cells(I + 2, 2)

A(1, 1) = 2 * (X1 - X2)
A(1, 2) = 2 * (Y1 - Y2)
A(2, 1) = 2 * (X1 - X3)
A(2, 2) = 2 * (Y1 - Y3)
B(1, 1) = (X1 ^ 2 - X2 ^ 2) + (Y1 ^ 2 - Y2 ^ 2)
B(2, 1) = (X1 ^ 2 - X3 ^ 2) + (Y1 ^ 2 - Y3 ^ 2)

.Cells(I, 6) = EquationAnswer(A, B)(1, 1)
.Cells(I, 7) = EquationAnswer(A, B)(2, 1)


I = I + 2
Loop Until I = 1101
End With
End Sub


Function EquationAnswer(ByVal A As Variant, ByVal B As Variant) As Variant
On Error GoTo ErrHndl
With WorksheetFunction
EquationAnswer = .MMult(.MInverse(A), B)
End With
On Error GoTo 0
Exit Function
ErrHndl:
EquationAnswer = CVErr(xlErrValue)
On Error GoTo 0
End Function

後記：尚、別の方法は下記ＵＲＬに記載してあります。
http://m-sudo.blogspot.com/2009/08/blog-post_3480.html