<form action="/action_page.php"

  oninput="x.value=parseInt(a.value)+parseInt(b.value)">

  0

  <input type="range" id="a" name="a" value="50">

  100 +

  <input type="number" id="b" name="b" value="50">

  =

  <output name="x" for="a b"></output>

  <br><br>

  <input type="submit"  value="Xác nhận">

</form>

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