Question

用VB编写程序,用二分法求方程y=x^3-x-1在(0,1)之间的根

Answer 1

二分法一般要先找到端点值异号的区间，然后再应用二分法。
这题在f(0)=-1<0,f(1)=-1<0, f(2)=5>0, 在（0，1）间f(0),f(1)同号，没有根在（0，1），而在(1，2）有根，程序可如下：
Function root() As Double
Dim x1 As Double, x2 As Double, x As Double, f1 As Double, f2 As Double, f As Double
x1 = 1
x2 = 2

f1 = x1 * x1 * x1 - x1 - 1
f2 = x2 * x2 * x2 - x2 - 1
Do
x = (x1 + x2) / 2
f = x * x * x - x - 1

If f > 0 Then
x2 = x
f2 = f
Else
x1 = x
f1 = f
End If
Loop Until Abs(x1 - x2) < 0.0000000001
root = x
End Function

Answer 2

二分法一般要先找到端点值异号的区间，然后再应用二分法。
这题在f(0)=-1<0,f(1)=-1<0, f(2)=5>0, 在（0，1）间f(0),f(1)同号，没有根在（0，1），而在(1，2）有根，程序可如下：
Function root() As Double
Dim x1 As Double, x2 As Double, x As Double, f1 As Double, f2 As Double, f As Double
x1 = 1
x2 = 2

f1 = x1 * x1 * x1 - x1 - 1
f2 = x2 * x2 * x2 - x2 - 1
Do
x = (x1 + x2) / 2
f = x * x * x - x - 1

If f > 0 Then
x2 = x
f2 = f
Else
x1 = x
f1 = f
End If
Loop Until Abs(x1 - x2) < 0.0000000001
root = x
End Function