<form name="form" onSubmit="return rechnen()">
  <input type="radio" name="auswahl" value="Addition" required> Addition<br>
  <input type="radio" name="auswahl" value="Subtraktion" required> Subtraktion<br>
  <input type="radio" name="auswahl" value="Multiplikation" required> Multiplikation<br>
  <input type="radio" name="auswahl" value="Division" required> Division<br>
  <input type="number" name="wert1" value="" readonly>
  <input type="text" name="op" value="" readonly>
  <input type="number" name="wert2" value="" readonly>
  =
  <input type="number" name="ergebnis" value="">
  <input type="submit" value="Aufgabe generieren">
</form>