Поиск максимального и минимального значения в массиве: исправить код - VB

Dim Xmax As Double, Xmin As Double, a As Double, b As Double, x As Double, h As Double
 
Private Sub Command1_Click()
 
a = 20
b = 21
n = Val(Text1)
 
Xmax = Xmin = a
 
 h = (b - a) / n
 x = a
 
 For i = 1 To n - 1
 x = x + h
 If f(x) < f(Xmin) Then Xmin = x
 If f(x) > f(Xmax) Then Xmax = x
 Debug.Print i
 Next i
 
 Text2 = f(Xmin)
 Text3 = f(Xmax)
End Sub
 
Private Function f(x) As Double
f = Sqr((x - 15) / (x + 3)) + 2 * Cos(x) / Sin(2 * x)
End Function