.lesson-container {
    max-width: 1000px;
    margin: 2rem auto;
    padding: 0 1rem;
}
.lesson-header {
    background: linear-gradient(135deg, #e74c3c, #c0392b);
    color: white;
    padding: 2rem;
    border-radius: 30px;
    margin-bottom: 2rem;
    text-align: center;
}
.lesson-header .lesson-badge {
    background: #ffd700;
    color: #e74c3c;
    width: 60px;
    height: 60px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin: 0 auto 1rem auto;
    font-size: 1.8rem;
    font-weight: bold;
}
.lesson-header h1 {
    font-size: 2rem;
    margin-bottom: 0.5rem;
}
.video-section {
    background: white;
    border-radius: 25px;
    padding: 1.5rem;
    margin-bottom: 2rem;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
}
.video-section h2 {
    color: #e74c3c;
    margin-bottom: 1rem;
    border-right: 4px solid #e74c3c;
    padding-right: 1rem;
}
.video-placeholder {
    background: linear-gradient(135deg, #e74c3c, #c0392b);
    border-radius: 20px;
    padding: 3rem;
    text-align: center;
    color: white;
    cursor: pointer;
    transition: 0.3s;
}
.video-placeholder:hover {
    transform: scale(1.02);
}
.video-placeholder .play-icon {
    font-size: 4rem;
    margin-bottom: 1rem;
}
.content-section {
    background: white;
    border-radius: 25px;
    padding: 1.5rem;
    margin-bottom: 2rem;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
}
.content-section h2 {
    color: #e74c3c;
    margin-bottom: 1rem;
    border-right: 4px solid #e74c3c;
    padding-right: 1rem;
}
.content-section h3 {
    color: #2a5298;
    margin: 1.5rem 0 0.5rem 0;
}
.formula-box {
    background: #2a5298;
    color: white;
    padding: 1rem;
    border-radius: 15px;
    margin: 1rem 0;
    text-align: center;
}
.info-box {
    background: #fff3cd;
    border-right: 4px solid #ffc107;
    padding: 1rem;
    border-radius: 15px;
    margin: 1rem 0;
}
.example-box {
    background: #e8f0fe;
    padding: 1rem;
    border-radius: 15px;
    margin: 1rem 0;
}
.rule-box {
    background: #f0f2f5;
    padding: 1rem;
    border-radius: 15px;
    margin: 1rem 0;
    border-right: 4px solid #e74c3c;
}
.rule-box strong {
    color: #e74c3c;
    display: block;
    margin-bottom: 0.5rem;
}
.back-button {
    text-align: center;
    margin: 2rem 0;
}
.back-button a {
    display: inline-block;
    background: #2a5298;
    color: white;
    padding: 0.8rem 1.5rem;
    border-radius: 40px;
    text-decoration: none;
    transition: 0.3s;
}
.back-button a:hover {
    background: #e74c3c;
    transform: scale(1.05);
}
@media (max-width: 768px) {
    .lesson-header h1 {
        font-size: 1.5rem;
    }
    .video-placeholder {
        padding: 1.5rem;
    }
}