二分法.frm

Visual Basic课程举例1 有很好的例题

VERSION 5.00
Begin VB.Form Form1 
   Caption         =   "二分法解方程"
   ClientHeight    =   3975
   ClientLeft      =   60
   ClientTop       =   345
   ClientWidth     =   7080
   BeginProperty Font 
      Name            =   "Arial Black"
      Size            =   21.75
      Charset         =   0
      Weight          =   400
      Underline       =   0   'False
      Italic          =   0   'False
      Strikethrough   =   0   'False
   EndProperty
   LinkTopic       =   "Form1"
   ScaleHeight     =   3975
   ScaleWidth      =   7080
   StartUpPosition =   3  '窗口缺省
   Begin VB.CommandButton CmdRoot 
      Caption         =   "开始求根"
      Height          =   615
      Left            =   2040
      TabIndex        =   5
      Top             =   3240
      Width           =   3495
   End
   Begin VB.TextBox Text2 
      Height          =   735
      Left            =   4800
      TabIndex        =   3
      Text            =   "2.5"
      Top             =   840
      Width           =   1335
   End
   Begin VB.TextBox Text1 
      Height          =   735
      Left            =   1560
      TabIndex        =   1
      Text            =   "2"
      Top             =   840
      Width           =   1335
   End
   Begin VB.Label Label3 
      Caption         =   "Sin(x)*2+3*Cos(x/5)-x^2=0"
      Height          =   615
      Left            =   240
      TabIndex        =   6
      Top             =   120
      Width           =   6495
   End
   Begin VB.Label lblRoot 
      Height          =   495
      Left            =   480
      TabIndex        =   4
      Top             =   2280
      Width           =   6375
   End
   Begin VB.Label Label2 
      Caption         =   "X2:"
      Height          =   495
      Left            =   3600
      TabIndex        =   2
      Top             =   840
      Width           =   975
   End
   Begin VB.Label Label1 
      Caption         =   "X1:"
      Height          =   495
      Left            =   360
      TabIndex        =   0
      Top             =   840
      Width           =   975
   End
End
Attribute VB_Name = "Form1"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = True
Attribute VB_Exposed = False
'二分法解方程，编制：曹新国，2004-3-14
'最后修改于 2006-4-5

Private Sub cmdRoot_Click()
'二分法求f(x)=Sin(x)*2+3*Cos(x/5)-x^2在区间[2，2.5]内的一个实根

    Dim x1 As Double, x2 As Double, x As Double
    x1 = Val(Text1) ' InputBox("输入区间x1的值", "输入框 ")
    x2 = Val(Text2) ' InputBox("输入区间x2的值", "输入框")
    x = x1
    f1 = Sin(x) * 2 + 3 * Cos(x / 5) - x ^ 2
    x = x2
    f2 = Sin(x) * 2 + 3 * Cos(x / 5) - x ^ 2
    If f1 * f2 < 0 Then
        Do
            x = (x1 + x2) / 2
            f = Sin(x) * 2 + 3 * Cos(x / 5) - x ^ 2
            If (f * f1) >= 0 Then
                x1 = x
                f1 = f
            End If
            If (f * f1) <= 0 Then
                x2 = x
                f2 = f
            End If
        Loop While Abs(x1 - x2) > 1E-20
        x = (x1 + x2) / 2

        lblRoot = "x=" & Format(x)
    Else
        MsgBox "方程在此区间内无根，请重新输入区间"
    End If
End Sub