vb计算多边形面积

Rem 凸n变形可以看成是n-1个三角形的面积
Rem 三角形的面积S=sqr(p*(p-a)*(p-b)*(p-c))，其中，p=(a+b+c)/2，a、b、c为边长，边长=sqr((x1-x2)^2+(y1-y2)^2)，而(x1,y1)和(x2,y2)分别为两点的坐标值

Rem 以下代码为求任意多边形的面积，边数由用户自行决定

Private Sub Command1_Click()
Dim n, s, i, x(), y(), a, b, c, p
k:
n = Int(Val(Trim(InputBox("请输入多边形的边数：", "多边形面积", 3))))
If n < 3 Then MsgBox "边数必须≥3！请重试！", , "多边形面积": GoTo k
On Error Resume Next

ReDim x(1 To n), y(1 To n)
s = 0
For i = 1 To n
x(i) = Val(Trim(InputBox("请输入" & IIf(n = 3, "三角形", n & "边形") & "第" & i & "个点的横坐标:")))
y(i) = Val(Trim(InputBox("请输入" & IIf(n = 3, "三角形", n & "边形") & "第" & i & "个点的横坐标:")))
If i > 1 Then Line (x(i), y(i))-(x(i - 1), y(i - 1))
Next
For i = 2 To n - 1
a = Sqr((x(1) - x(i)) ^ 2 + (y(1) - y(i)) ^ 2)
b = Sqr((x(1) - x(