Не сравниваются значения переменных - VB

While ((b - a) / 2) > oshibka
sigma = (b - a) * 0.1
x1 = (b + a - sigma) / 2
x2 = (b + a + sigma) / 2
znach_fun1 = k1 + k2 * x1 + k3 * x1 ^ 2 + k4 * x1 ^ 3
zhach_fun2 = k1 + k2 * x2 + k3 * x2 ^ 2 + k4 * x2 ^ 3
If znach_fun1 >= zhach_fun2 Then
b = x2
Else
a = x1
End If
iter = iter + 1
Wend

        While (b - a) / 2 > oshibka
            sigma = (b - a) * 0.1
            x1 = (b + a - sigma) / 2
            x2 = (b + a + sigma) / 2
            znach_fun1 = k1 + k2 * x1 + k3 * x1 ^ 2 + k4 * x1 ^ 3
            zhach_fun2 = k1 + k2 * x2 + k3 * x2 ^ 2 + k4 * x2 ^ 3
            If znach_fun1 >= zhach_fun2 Then
                b = x2
            Else
                a = x1
            End If
            iter = iter + 1
        End While

Dim fun, k1, k2, k3, k4, oshibka As String
Dim sigma, znach_fun1, zhach_fun2, x_iscom, zhach_fun_x_iscom, x1, x2, a, b, prov As Double
Dim iter As Integer
 
Private Sub Command1_Click()
 
k1 = CDbl(Text1)
k2 = CDbl(Text2)
k3 = CDbl(Text3)
k4 = CDbl(Text4)
a = CDbl(Text5)
b = CDbl(Text6)
oshibka = CDbl(Text7)
iter = 1
 
While ((b - a) / 2) > oshibka
sigma = (b - a) * 0.1
x1 = (b + a - sigma) / 2
x2 = (b + a + sigma) / 2
znach_fun1 = k1 + k2 * x1 + k3 * x1 ^ 2 + k4 * x1 ^ 3
zhach_fun2 = k1 + k2 * x2 + k3 * x2 ^ 2 + k4 * x2 ^ 3
If znach_fun1 >= zhach_fun2 Then
b = x2
Else
a = x1
End If
iter = iter + 1
Wend
 
x_iscom = (a + b) / 2
zhach_fun_x_iscom = k1 + k2 * x_iscom + k3 * x_iscom ^ 2 + k4 * x_iscom ^ 3
 
k1 = Text1
k2 = Text2
k3 = Text3
k4 = Text4
oshibka = Text7
 
fun = "(" + k1 + ") + (" + k2 + ")x + (" + k3 + ")x^2 + (" + k4 + ")x^3"
 
Text8 = "Г‡Г*Г¤Г*Г*Г*Г*Гї ГґГіГ*êöèÿ: " + fun + vbCrLf + "Г‡Г*Г¤Г*Г*Г*Г*Гї îøèáêГ*: " + oshibka + vbCrLf + "Êîëè÷åñòâî ГЁГІГҐГ°Г*öèé: " + CStr(iter) + vbCrLf + "Ïîëó÷åГ*Г*ûé Г¬Г*êñèìóì ГґГіГ*êöèè: " + CStr(zhach_fun_x_iscom) + vbCrLf + "Òî÷êГ* Г*Г*áëþäåГ*ГЁГї Г¬Г*êñèìóìГ*: " + CStr(x_iscom)

End Sub