Sub s() Dim l#, u#, m# l = 0 u = 1 Do While u - l > 10 ^ -10 m = (u + l) / 2 y = m ^ 4 - m ^ 3 - 4 * m ^ 2 + 1 If y = 0 Then Exit Do If y > 0 Then l = m Else u = m End If Loop MsgBox "x=" & mEnd Sub