Using an equation as part of an input for a VBA function? - vba

Question

I'm currently writing VBA code for a lab for one of my engineering classes. The goal is to use a single function to approximate (Trapezoidal Rule) the integral of a given equation. The equation will need to change at some point. I'm new to programming so I'm planning on seeing some sort of simple logic error. Here's what I currently have:
Function Trapezoidal(ByVal sFx As String, ByVal A As Double, _
ByVal B As Double, ByVal N As Integer) As Double
' Calculates the area under the curve using the Trapezoidal rule.
'
' Parameters:
'
' sFx - String expression that has the function to be
' integrated. The variable X as to appear as $X.
' An example is: $X*LOG(X$) which is an expression for function
' f(x)=x*ln(x)
' A, B - Lower and Upper limit for the integral.
' N - The number of integration intervals.
'
Dim Sum As Double, DeltaX As Double, X As Double
Dim I As Integer
Sum = 0
DeltaX = (B - A) / N
X = A
For I = 1 To N
Sum = Sum + (Fx(sFx, X) + Fx(sFx, X + DeltaX)) / 2
X = X + DeltaX
Next I
Sum = DeltaX * Sum
Trapezoidal = Sum
End Function
I've sort of Frankensteined a few pieces of code together in my research.Something clearly isn

Answer 1

Sub Tester()
Debug.Print Trapezoidal("$X*LN($X)", 1, 20, 5)
End Sub
Function Trapezoidal(ByVal sFx As String, ByVal A As Double, _
ByVal B As Double, ByVal N As Integer) As Double
' Calculates the area under the curve using the Trapezoidal rule.
'
' Parameters:
'
' sFx - String expression that has the function to be
' integrated. The variable X as to appear as $X.
' An example is: $X*LOG(X$) which is an expression for function
' f(x)=x*ln(x)
' A, B - Lower and Upper limit for the integral.
' N - The number of integration intervals.
'
Dim Sum As Double, DeltaX As Double, X As Double
Dim I As Integer, f As String
Sum = 0
DeltaX = (B - A) / N
X = A
For I = 1 To N
'Sum = Sum + (Fx(sFx, X) + Fx(sFx, X + DeltaX)) / 2
f = "(" & Replace(sFx, "$X", X) & " + " & Replace(sFx, "$X", X + DeltaX) & ")/2"
Debug.Print f
Sum = Sum + Application.Evaluate(f)
X = X + DeltaX
Next I
Sum = DeltaX * Sum
Trapezoidal = Sum
End Function
Debug output:
(1*LN(1) + 4.8*LN(4.8))/2
(4.8*LN(4.8) + 8.6*LN(8.6))/2
(8.6*LN(8.6) + 12.4*LN(12.4))/2
(12.4*LN(12.4) + 16.2*LN(16.2))/2
(16.2*LN(16.2) + 20*LN(20))/2
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