Integrating Over an Interval with VBA

[FreshSaladBar]

I am writing a VBA script to integrate over an interval in Excel. I have begun with a simple position and velocity formula to test my code then expanded on it. Unfortunately, it is not working properly as the outcome I am recieving is .044 while the expected outcome using MATHCAD is .127.

Dim dIdt As Double
Dim Isnp As Double
Dim Tau As Double


Dim A As Double
Dim B As Double
Dim C As Double
Dim D As Double


Function Iscr(t)
dIdt = Range("B22").Value
Isnp = Range("B27").Value
Tau = Range("B26").Value
Iscr = (dIdt * t) + (Isnp * (Exp((t) / (-1 * Tau))))
End Function


Function Econd1(t)
A = Range("B28").Value
B = Range("B29").Value
C = Range("B30").Value
D = Range("B31").Value
Econd1 = A + (B * (Log(Iscr(t)))) + (C * (Iscr(t))) + (D * (Iscr(t) ^ (1 / 2)) * Iscr(t))
End Function


Function Deriv(x0, t1, t2)


'define range of integral
int_range = t2 - t1


'discretize the integral into n slices dt wide
n = 5000
Dt = int_range / n


'initialize variables
ta = t1
tb = ta + Dt
Deriv = x0


'calculate areas using trapezoidal rule


'sum area under curve of each slice
For j = 1 To n
Deriv = Deriv + (tb - ta) * (Econd1(ta) + Econd1(tb)) / 2
ta = tb
tb = ta + Dt
Next


End Function

I'm not sure where this project has gone wrong considering it is working properly with my simpler function.