* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --primary-color: #2563eb;
  --primary-hover: #1d4ed8;
  --secondary-color: #64748b;
  --success-color: #16a34a;
  --warning-color: #ea580c;
  --error-color: #dc2626;
  --background: #ffffff;
  --surface: #f8fafc;
  --text-primary: #1e293b;
  --text-secondary: #64748b;
  --border: #e2e8f0;
  --shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1);
  --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  line-height: 1.6;
  color: var(--text-primary);
  background-color: var(--background);
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 20px;
}

/* Header */
.header {
  background: var(--background);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 100;
}

.nav-container {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 1rem 20px;
  max-width: 1200px;
  margin: 0 auto;
}

.logo {
  display: flex;
  align-items: center;
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--primary-color);
  text-decoration: none;
}

.logo-icon {
  background: var(--primary-color);
  color: white;
  width: 32px;
  height: 32px;
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  margin-right: 8px;
  font-size: 1.2rem;
}

.nav-menu {
  display: flex;
  gap: 2rem;
}

.nav-link {
  color: var(--text-secondary);
  text-decoration: none;
  font-weight: 500;
  transition: color 0.2s;
}

.nav-link:hover {
  color: var(--primary-color);
}

/* Hero */
.hero {
  background: linear-gradient(135deg, var(--surface) 0%, var(--background) 100%);
  padding: 4rem 0 2rem;
  text-align: center;
}

.hero-title {
  font-size: 3rem;
  font-weight: 800;
  color: var(--text-primary);
  margin-bottom: 1rem;
}

.hero-subtitle {
  font-size: 1.25rem;
  color: var(--text-secondary);
  max-width: 600px;
  margin: 0 auto 2rem;
}

.hero-features {
  display: flex;
  justify-content: center;
  gap: 2rem;
  flex-wrap: wrap;
}

.feature {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  color: var(--text-secondary);
  font-weight: 500;
}

.feature-icon {
  font-size: 1.2rem;
}

/* Calculator */
.calculator-section {
  padding: 3rem 0;
}

.calculator-tabs {
  display: flex;
  justify-content: center;
  margin-bottom: 2rem;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.tab-button {
  padding: 0.75rem 1.5rem;
  background: var(--surface);
  border: 1px solid var(--border);
  color: var(--text-secondary);
  cursor: pointer;
  border-radius: 8px;
  font-weight: 500;
  transition: all 0.2s;
}

.tab-button:hover {
  background: var(--background);
  border-color: var(--primary-color);
}

.tab-button.active {
  background: var(--primary-color);
  color: white;
  border-color: var(--primary-color);
}

.calculator-tab {
  display: none;
}

.calculator-tab.active {
  display: block;
}

.calculator-card {
  background: var(--background);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 2rem;
  box-shadow: var(--shadow);
  max-width: 500px;
  margin: 0 auto;
}

.calculator-card h2 {
  margin-bottom: 1.5rem;
  color: var(--text-primary);
  font-size: 1.5rem;
  text-align: center;
}

.input-group {
  margin-bottom: 1.5rem;
}

.input-group label {
  display: block;
  margin-bottom: 0.5rem;
  font-weight: 500;
  color: var(--text-primary);
}

.input-group input {
  width: 100%;
  padding: 0.75rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 1rem;
  transition: border-color 0.2s;
}

.input-group input:focus {
  outline: none;
  border-color: var(--primary-color);
  box-shadow: 0 0 0 3px rgb(37 99 235 / 0.1);
}

.radio-group {
  display: flex;
  gap: 1rem;
  margin-bottom: 1.5rem;
}

.radio-group label {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  cursor: pointer;
}

.calculate-btn {
  width: 100%;
  padding: 0.875rem;
  background: var(--primary-color);
  color: white;
  border: none;
  border-radius: 8px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background-color 0.2s;
}

.calculate-btn:hover {
  background: var(--primary-hover);
}

.result {
  margin-top: 1.5rem;
  padding: 1rem;
  background: var(--surface);
  border-radius: 8px;
  font-size: 1.25rem;
  font-weight: 600;
  text-align: center;
  color: var(--success-color);
}

.explanation {
  margin-top: 1rem;
  padding: 1rem;
  background: #f0f9ff;
  border-left: 4px solid var(--primary-color);
  border-radius: 8px;
  color: var(--text-secondary);
  line-height: 1.6;
}

/* New Sections Styles */
.section-title {
  text-align: center;
  margin-bottom: 3rem;
  font-size: 2.25rem;
  color: var(--text-primary);
  font-weight: 700;
}

/* Info Section */
.info-section {
  padding: 4rem 0;
  background: var(--surface);
}

.info-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 2rem;
}

.info-card {
  background: var(--background);
  padding: 2rem;
  border-radius: 12px;
  box-shadow: var(--shadow);
  text-align: center;
  transition: transform 0.2s, box-shadow 0.2s;
}

.info-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
}

.info-icon {
  font-size: 3rem;
  margin-bottom: 1rem;
}

.info-card h3 {
  margin-bottom: 1rem;
  color: var(--text-primary);
  font-size: 1.25rem;
}

.info-card p {
  color: var(--text-secondary);
  line-height: 1.7;
}

/* Guide Section */
.guide-section {
  padding: 4rem 0;
  background: var(--background);
}

.guide-content {
  max-width: 900px;
  margin: 0 auto;
}

.guide-content h3 {
  color: var(--primary-color);
  margin: 2rem 0 1rem;
  font-size: 1.75rem;
}

.guide-content h4 {
  color: var(--text-primary);
  margin: 1.5rem 0 0.75rem;
  font-size: 1.25rem;
}

.guide-content p, .guide-content li {
  color: var(--text-secondary);
  line-height: 1.7;
  margin-bottom: 1rem;
}

.guide-content ul {
  margin: 1rem 0 1.5rem 1.5rem;
}

/* Examples Section */
.examples-section {
  padding: 4rem 0;
  background: var(--surface);
}

.examples-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 2rem;
}

.example-category {
  background: var(--background);
  padding: 2rem;
  border-radius: 12px;
  box-shadow: var(--shadow);
}

.example-category h3 {
  color: var(--primary-color);
  margin-bottom: 1.5rem;
  font-size: 1.5rem;
}

.example-list {
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

.example-item {
  border-left: 4px solid var(--primary-color);
  padding-left: 1rem;
}

.example-item h4 {
  color: var(--text-primary);
  margin-bottom: 0.5rem;
  font-size: 1.1rem;
}

.example-item p {
  color: var(--text-secondary);
  line-height: 1.6;
  margin-bottom: 0.5rem;
}

.calculation {
  font-family: 'Monaco', 'Menlo', monospace;
  background: #f0f9ff;
  padding: 0.5rem;
  border-radius: 4px;
  color: var(--primary-color);
  font-weight: 600;
}

/* Tips Section */
.tips-section {
  padding: 4rem 0;
  background: var(--background);
}

.tips-content {
  max-width: 1000px;
  margin: 0 auto;
}

.tip-category {
  margin-bottom: 3rem;
}

.tip-category h3 {
  color: var(--primary-color);
  margin-bottom: 1rem;
  font-size: 1.75rem;
}

.tips-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 1.5rem;
  margin: 2rem 0;
}

.tip-card {
  background: #f0f9ff;
  padding: 1.5rem;
  border-radius: 8px;
  border-left: 4px solid var(--primary-color);
}

.tip-card h4 {
  color: var(--text-primary);
  margin-bottom: 0.75rem;
  font-size: 1.1rem;
}

.tip-card p {
  color: var(--text-secondary);
  line-height: 1.6;
  margin-bottom: 0.5rem;
}

.combination-examples {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 2rem;
  margin: 2rem 0;
}

.combo-example {
  background: var(--surface);
  padding: 1.5rem;
  border-radius: 8px;
}

.combo-example h4 {
  color: var(--primary-color);
  margin-bottom: 0.75rem;
}

.combo-example ul {
  margin: 0.5rem 0 0.5rem 1.5rem;
}

.verification-tips {
  background: #fef3c7;
  padding: 1.5rem;
  border-radius: 8px;
  border-left: 4px solid var(--warning-color);
}

.verification-tips p {
  font-weight: 600;
  margin-bottom: 1rem;
}

.verification-tips h4 {
  color: var(--warning-color);
  margin: 1rem 0 0.5rem;
}

/* Business Section */
.business-section {
  padding: 4rem 0;
  background: var(--surface);
}

.business-applications {
  max-width: 1100px;
  margin: 0 auto;
}

.business-category {
  margin-bottom: 3rem;
}

.business-category h3 {
  color: var(--primary-color);
  margin-bottom: 2rem;
  font-size: 1.75rem;
  text-align: center;
}

.business-examples {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
  gap: 2rem;
}

.business-example {
  background: var(--background);
  padding: 2rem;
  border-radius: 12px;
  box-shadow: var(--shadow);
}

.business-example h4 {
  color: var(--text-primary);
  margin-bottom: 1rem;
  font-size: 1.25rem;
}

.business-example p {
  color: var(--text-secondary);
  line-height: 1.7;
  margin-bottom: 0.75rem;
}

.formula {
  font-family: 'Monaco', 'Menlo', monospace;
  background: #f0f9ff;
  padding: 0.75rem;
  border-radius: 6px;
  color: var(--primary-color);
  font-weight: 600;
  text-align: center;
  margin: 0.75rem 0;
}

/* Advanced Section */
.advanced-section {
  padding: 4rem 0;
  background: var(--background);
}

.advanced-content {
  max-width: 1000px;
  margin: 0 auto;
}

.advanced-topic {
  margin-bottom: 3rem;
  background: var(--surface);
  padding: 2rem;
  border-radius: 12px;
}

.advanced-topic h3 {
  color: var(--primary-color);
  margin-bottom: 1rem;
  font-size: 1.75rem;
}

.advanced-topic p {
  color: var(--text-secondary);
  line-height: 1.7;
  margin-bottom: 1rem;
}

.example-detailed {
  background: var(--background);
  padding: 1.5rem;
  border-radius: 8px;
  margin: 1rem 0;
  border-left: 4px solid var(--success-color);
}

.example-detailed h4 {
  color: var(--success-color);
  margin-bottom: 0.75rem;
}

.trend-examples, .statistics-examples {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 1.5rem;
  margin: 1.5rem 0;
}

.trend-example, .stat-example {
  background: var(--background);
  padding: 1.5rem;
  border-radius: 8px;
  border-left: 4px solid var(--primary-color);
}

.trend-example h4, .stat-example h4 {
  color: var(--primary-color);
  margin-bottom: 0.75rem;
}

.cumulative-example {
  background: #fef2f2;
  padding: 1.5rem;
  border-radius: 8px;
  border-left: 4px solid var(--error-color);
  margin: 1rem 0;
}

.cumulative-example h4 {
  color: var(--error-color);
  margin-bottom: 0.75rem;
}

/* Content Section */
.content-section {
  padding: 4rem 0;
  background: var(--surface);
}

.content-section h2 {
  text-align: center;
  margin-bottom: 3rem;
  font-size: 2rem;
  color: var(--text-primary);
}

.articles-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 2rem;
}

.article-card {
  background: var(--background);
  border-radius: 12px;
  padding: 1.5rem;
  box-shadow: var(--shadow);
  transition: transform 0.2s, box-shadow 0.2s;
}

.article-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-lg);
}

.article-card h3 {
  margin-bottom: 0.75rem;
}

.article-card h3 a {
  color: var(--text-primary);
  text-decoration: none;
  font-size: 1.25rem;
}

.article-card h3 a:hover {
  color: var(--primary-color);
}

.article-card p {
  color: var(--text-secondary);
  line-height: 1.6;
}

/* Footer */
.footer {
  background: var(--text-primary);
  color: white;
  padding: 3rem 0 1rem;
}

.footer-content {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 2rem;
  margin-bottom: 2rem;
}

.footer-section h4 {
  margin-bottom: 1rem;
  color: white;
}

.footer-section p {
  color: #94a3b8;
  line-height: 1.6;
}

.footer-section ul {
  list-style: none;
}

.footer-section ul li {
  margin-bottom: 0.5rem;
}

.footer-section ul li a {
  color: #94a3b8;
  text-decoration: none;
  transition: color 0.2s;
}

.footer-section ul li a:hover {
  color: white;
}

.footer-bottom {
  text-align: center;
  padding-top: 2rem;
  border-top: 1px solid #374151;
  color: #94a3b8;
}

/* Responsive Design */
@media (max-width: 768px) {
  .nav-menu {
    flex-direction: column;
    gap: 1rem;
  }

  .hero-title {
    font-size: 2rem;
  }

  .hero-subtitle {
    font-size: 1.1rem;
  }

  .hero-features {
    flex-direction: column;
    gap: 1rem;
  }

  .calculator-tabs {
    flex-direction: column;
  }

  .tab-button {
    width: 100%;
  }

  .calculator-card {
    margin: 0 1rem;
    padding: 1.5rem;
  }

  .section-title {
    font-size: 1.75rem;
  }

  .info-grid {
    grid-template-columns: 1fr;
  }

  .examples-grid {
    grid-template-columns: 1fr;
  }

  .tips-grid {
    grid-template-columns: 1fr;
  }

  .combination-examples {
    grid-template-columns: 1fr;
  }

  .business-examples {
    grid-template-columns: 1fr;
  }

  .trend-examples, .statistics-examples {
    grid-template-columns: 1fr;
  }

  .articles-grid {
    grid-template-columns: 1fr;
  }

  .footer-content {
    grid-template-columns: 1fr;
    text-align: center;
  }
}