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Option Explicit

Sub EnterSquareRoot()
Dim Num As Variant

    If TypeName(Selection) <> "Range" Then
        MsgBox "Select a cell for the result."
        Exit Sub
    End If
    
Num = InputBox("Enter a value")
    If Not IsNumeric(Num) Then
        MsgBox ("You must enter a number.")
        Exit Sub
    End If
    
    If Num < 0 Then
        MsgBox ("You must enter a positive number.")
        Exit Sub
    End If
    
ActiveCell.Value = Sqr(Num)
  
End Sub