body { 
    font-family: Arial, sans-serif; 
    margin: 0; 
    padding: 0; 
    background: #f4f4f4; 
  }
  header {
    color: #fff;
    padding: 10px 20px;
    display: flex;
    align-items: center;
    justify-content: space-between;
  }
  header h1 { margin: 0; font-size: 1.5em; }
  nav a {
    color: #fff;
    text-decoration: none;
    margin-right: 15px;
  }
  .container {
    display: flex;
    height: calc(100vh - 50px);
  }
  .sidebar {
    width: 250px;
    background: #fff;
    padding: 15px;
    border-right: 1px solid #ddd;
    overflow-y: auto;
  }
  .sidebar h3 { margin-top: 0; }
  .sidebar ul { list-style: none; padding-left: 0; }
  .sidebar ul li { margin-bottom: 10px; }
  .sidebar ul li a { color: #333; text-decoration: none; }
  .main {
    flex: 1;
    padding: 20px;
    overflow-y: auto;
  }
  .main h2 { margin-top: 0; }
  .section {
    border: 1px solid #ddd;
    border-radius: 5px;
    margin-bottom: 20px;
    background: #fff;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
  }
  .section-header {
    background: #eee;
    padding: 10px;
    cursor: pointer;
    font-weight: bold;
    user-select: none;
  }
  .section-content {
    display: none;
    padding: 15px;
  }
  .section-content p {
    margin-top: 0;
  }
  pre {
    background: #1e1e1e;
    color: #fff;
    padding: 10px;
    overflow-x: auto;
    border-radius: 3px;
  }

p, a{
    font-family: 'Times New Roman', Times, serif;
    font-size: 18px;
}
/* tag */
.blog-tags {
    display: flex;
    gap: 5px;
    margin-top: -10px;
}

.tag {
    height: 20px;
    width: auto;
    background-color: #aed6f1;
    color: white;
    padding: 5px 10px;
    border-radius: 5px;
    font-family: 'Times New Roman', Times, serif;
    font-size: 15px;
}
/* cong thuc */
.formula {
    font-size: 20px;  
    font-weight: bold;
    font-family: 'Times New Roman', Times, serif;
    color: #d63384;  
    background: #fff5f8;  
    padding: 2px 6px; 
    border-radius: 3px; 
    border-left: 3px solid #d63384;
    display: inline;
    margin-top: 15px;
}
/* code */
.stylecode {
    color: #a9dfbf; 
    padding: 2px 6px; 
    border-radius: 4px; 
    font-family: Consolas, Monaco, 'Courier New', monospace; 
}
/* solution card */
.solution-card {
  background: #f8f9fa;
  border-radius: 8px;
  box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1);
  margin: 15px 0;
  padding: 15px;
  border-left: 5px solid #aed6f1;
}
.solution-header {
  font-size: 18px;
  font-weight: bold;
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
}
.solution-content {
  display: none;
  margin-top: 10px;
}
.solution-card.open .solution-content {
  display: block;
}
.toggle-icon {
  font-size: 18px;
  transition: transform 0.3s ease;
}
.solution-card.open .toggle-icon {
  transform: rotate(180deg);
}


.fontext *:not(.stylecode) {
  font-family: "Times New Roman", Times, serif;
}

/* copy code */
.copy-btn {
  position: absolute;
  right: 10px;
  top: 10px;
  background: #3498db;
  color: white;
  border: none;
  padding: 5px 10px;
  font-size: 14px;
  cursor: pointer;
  border-radius: 5px;
  transition: 0.3s;
}

.copy-btn:hover {
  background: #2980b9;
}

.solution-content {
  position: relative;
}