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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Generate triangular numbers</title>
</head>
<body>
  <h1>Triangular numbers for k</h1>
  <p>Edit <code>kMax</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      var kMax = 9;
      var lines = [];
      var k;
      lines.push("Triangular numbers for k = 1 to " + kMax + ":");
      for (k = 1; k <= kMax; k++) {
        var value = Math.floor((k * (k + 1)) / 2);
        lines.push("T_" + k + " = " + k + "*" + (k + 1) + "/2 = " + value);
      }
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

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JavaScript Triangular Number — Generate Formula (Try It)