dropdown.php

<div id="wrapper">
<ul class="sidebar navbar-nav">
                <li class="nav-item active">
                <a class="nav-link" href="index.php">
                    <i class="fas fa-fw fa-tachometer-alt"></i>
                    <span>Dashboard</span>
                </a>
                </li>
                <li class="nav-item dropdown">
                    <a class="nav-link dropdown-toggle" href="#" id="pagesDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                        <i class="fas fa-fw fa-folder"></i>
                        <span>Pages</span>
                    </a>
                    <div class="dropdown-menu" aria-labelledby="pagesDropdown">
                        <h6 class="dropdown-header">Login Screens:</h6>
                        <a class="dropdown-item" href="login.html">Login</a>
                        <a class="dropdown-item" href="register.html">Register</a>
                        <a class="dropdown-item" href="forgot-password.html">Forgot Password</a>
                        <div class="dropdown-divider"></div>
                        <h6 class="dropdown-header">Other Pages:</h6>
                        <a class="dropdown-item" href="404.html">404 Page</a>
                        <a class="dropdown-item" href="blank.html">Blank Page</a>
                    </div>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="baiso1.php">
                        <i class="fas fa-fw fa-chart-area"></i>
                        <span>Bài 1</span>
                    </a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="baiso2.php">
                        <i class="fas fa-fw fa-chart-area"></i>
                        <span>Bài 2</span>
                    </a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="baiso3.php">
                        <i class="fas fa-fw fa-chart-area"></i>
                        <span>Bài 3</span>
                    </a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="bai4.php">
                        <i class="fas fa-fw fa-chart-area"></i>
                        <span>Bai 4</span></a>
                </li>
                <li class="nav-item">
                <a class="nav-link" href="baiso5.php">
                    <i class="fas fa-fw fa-table"></i>
                    <span>Bai so 5</span></a>
                </li>
                <li class="nav-item">
                <a class="nav-link" href="baiso6.php">
                    <i class="fas fa-fw fa-table"></i>
                    <span>Bai so 6</span></a>
                </li>
                <li class="nav-item">
                <a class="nav-link" href="baitap.php">
                    <i class="fas fa-fw fa-table"></i>
                    <span>Bai tap</span></a>
                </li>
                <li class="nav-item">
                <a class="nav-link" href="displayBook.php">
                    <i class="fas fa-fw fa-table"></i>
                    <span>List Book</span></a>
                </li>
            </ul>