<header>
  <h2 class="title">Math.log2()</h2>
  <p class="description">Повертає базовий логарифм числа 2.</p>
</header>
<main>
  <div class="result">
    <label for="numberInput">Введіть число:</label>
    <input id="numberInput" type="number" placeholder="Введіть число" value="16">
    <button id="calculateBtn">Обчислити</button>
    <p>Результат: <span id="output"></span></p>
  </div>
</main>

body {
  font-size: 16px;
  line-height: 1.5;
  font-family: monospace;
}

header {
  background-color: #f1f1f1;
  margin-bottom: 25px;
  padding: 15px;
  -webkit-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  -moz-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
}

header h2.title {
  padding-bottom: 15px;
  border-bottom: 1px solid #999;
}

header p.description {
  font-style: italic;
  color: #222;
}

.result {
  background-color: #f8f8f8;
  padding: 15px;
  -webkit-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  -moz-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
}

input {
  margin-right: 15px;
}

button {
  background-color: #4CAF50;
  color: white;
  border: none;
  padding: 8px 15px;
  cursor: pointer;
  transition: 0.3s;
}

button:hover {
  background-color: #45a049;
}

document.getElementById("calculateBtn").addEventListener("click", function() {
  const number = parseFloat(document.getElementById("numberInput").value);
  const result = Math.log2(number);
  document.getElementById("output").innerText = result.toFixed(2);
});

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External JavaScript

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