'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！