'使用下面提供给你的函数可以做到这点！
'先定义2个结构Parent和Plus，把这2个定义放在代码段最前面
Private Type Parent
s As String
value As Double
End Type
Private Type Plus
s As String
value As Double
End Type
'下面是实现此功能的函数的定义，比较长
Public Function ValueOfExpression(ByVal Express As String) As Double
Dim Pa() As Parent, ParNum As Integer, Ps() As Plus, OperNum As Integer
Dim str0 As String
'按括号分解表达式 Express:Begin-----/*
Dim lenExp As Integer, Lenstr1 As Integer, i As Integer, j As Integer, k As Integer, str1 As String, str2 As String, intPar As Integer
Dim intStart As Integer, intEnd As Integer
lenExp = Len(Express)
For i = 1 To lenExp
If Mid(Express, i, 1) = "(" Then intPar = intPar + 1
Next
ParNum = intPar
ReDim Pa((intPar / 10 + 1) * 10)
For i = 1 To intPar
If intPar < 1 Then Exit For
For j = 1 To lenExp
If Mid(Express, j, 1) = ")" Then
str1 = Mid(Express, 1, j - 1)
Exit For
End If
Next
Lenstr1 = Len(str1)
For k = 1 To Lenstr1
If Mid(str1, Lenstr1 + 1 - k, 1) = "(" Then
Pa(i).s = Mid(str1, Lenstr1 + 2 - k)
Exit For
End If
Next
Express = Mid(Express, 1, Lenstr1 - k) & Chr(128) & CStr(i) & Mid(Express, j + 1)
lenExp = Len(Express)
Next
Pa(0).s = Express
'*/-----End
'按加减号进一步分解：Begin-----/*
Dim n As Integer, strLeft As String
For i = 0 To ParNum
k = 0
For j = 1 To Len(Pa(i).s)
str1 = Mid(Pa(i).s, j, 1)
If str1 = "+" Or str1 = "-" Then k = k + 1
Next
If k > OperNum Then OperNum = k
Next
ReDim Ps(ParNum, OperNum)
For i = 0 To ParNum
strLeft = Pa(i).s: n = 0: str2 = ""
Do
If Len(strLeft) = 0 Then Exit Do
For j = 1 To Len(strLeft)
str1 = Mid(strLeft, j, 1)
If str1 = "+" Or str1 = "-" Then
Ps(i, n).s = str2 & Mid(strLeft, 1, j - 1)
n = n + 1
str2 = IIf(str1 = "-", str1, "")
strLeft = Mid(strLeft, j + 1)
Exit For
End If
If j = Len(strLeft) Then
Ps(i, n).s = str2 & strLeft: j = 0
Exit For
End If
Next
Loop Until j = 0
Next
'*/-----End
'计算最后分成的多个简单表达式的值的总和,即表达式 Express 的值
Dim Total As Double, value As Double
For i = 1 To ParNum + 1
If i = ParNum + 1 Then i = 0
Total = 0
For j = 0 To OperNum
Express = Ps(i, j).s: value = 0
Dim lasti As Integer, operator As String
lenExp = Len(Express): lasti = 0: operator = ""
For k = 1 To lenExp
str0 = Mid(Express, k, 1)
If InStr("*/^", str0) > 0 Or k = lenExp Then
If k = lenExp Then k = k + 1
str1 = Mid(Express, lasti + 1, k - 1 - lasti)
Dim sign As Integer, Valstr1 As Double
If Mid(str1, 1, 1) = "-" Then
sign = -1
str1 = Mid(str1, 2)
Else
sign = 1
End If
n = InStr(1, "/sin" & Chr(128) & "/cos" & Chr(128) & "/tan" & Chr(128) & "/abs" & Chr(128) & "/atn" & Chr(128) & "/exp" & Chr(128) & "/int" & Chr(128) & "/fix" & Chr(128) & "/sgn" & Chr(128) & "/sqr" & Chr(128) & "/", "/" & Mid(str1, 1, 4) & "/")
If n > 0 Then
Valstr1 = Choose((n + 4) / 5, Sin(Pa(Val(Mid(str1, 5))).value), Cos(Pa(Val(Mid(str1, 5))).value), Tan(Pa(Val(Mid(str1, 5))).value), Abs(Pa(Val(Mid(str1, 5))).value), Atn(Pa(Val(Mid(str1, 5))).value), Exp(Pa(Val(Mid(str1, 5))).value), Int(Pa(Val(Mid(str1, 5))).value), Fix(Pa(Val(Mid(str1, 5))).value), Sgn(Pa(Val(Mid(str1, 5))).value), Sqr(Pa(Val(Mid(str1, 5))).value))
Else
n = InStr(1, "/lg" & Chr(128) & "/ln" & Chr(128) & "/", Mid(str1, 1, 3))
If n > 0 Then
Valstr1 = Choose((n + 3) / 4, Log(Pa(Val(Mid(str1, 4))).value) / Log(10), Log(Pa(Val(Mid(str1, 4))).value))
Else
If Mid(str1, 1, 1) = Chr(128) Then
Valstr1 = Pa(Val(Mid(str1, 2))).value
ElseIf Right(str1, 1) = "!" Then
If Val(str1) = 0 Then
Valstr1 = 1
Else
Valstr1 = 1
For n = 1 To Val(str1)
Valstr1 = Valstr1 * n
Next
End If
Else
Valstr1 = Val(str1)
End If
End If
End If
Valstr1 = Valstr1 * sign
Select Case operator
Case ""
value = Valstr1
Case "*"
value = value * Valstr1
Case "/"
value = value / Valstr1
Case "^"
value = value ^ Valstr1
End Select
lasti = k: operator = str0
End If
Next
Ps(i, j).value = value
Total = Total + Ps(i, j).value
Next
Pa(i).value = Total
If i = 0 Then Exit For
Next
ValueOfExpression = Pa(0).value
End Function
'OK，到这里结束了该函数的定义
'使用例子：
'A="5+5+5"
'Print ValueOfExpression(A)
'其它说明：该函数支持很多数学函数如sin、cos、tan等等，如：
'Print ValueOfExpression("sin(1)")
'注意后面要加括号
'该函数支持运算符+-*/^!，还有括号，^为乘幂，其优先级与*/相同，这是使用该函数需要注意的地方；!为阶乘
'自己试试就知道怎么回事了！