使用方程作为VBA函数的input的一部分？
我目前正在为我的一个工程类写实验室的VBA代码。 目标是使用单个函数来近似（梯形法则）给定方程的积分。 这个公式需要在某个时候改变。 我是编程新手，所以我打算看到某种简单的逻辑错误。 这是我现在有的东西：
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 在我的研究中，我已经搞清楚了一些代码。有些事情显然不是
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
debugging输出：
(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))/2 502.848119401941