:root {
      --primary: #4f46e5;
      --primary-dark: #3730a3;
      --primary-light: #eef2ff;
      --accent: #f59e0b;
      --green: #10b981;
      --red: #ef4444;
      --dark: #0f172a;
      --text: #334155;
      --muted: #64748b;
      --border: #e2e8f0;
      --bg: #f8fafc;
      --white: #ffffff;
      --radius: 18px;
      --shadow: 0 18px 45px rgba(15, 23, 42, .08);
    }

    * {
      box-sizing: border-box;
    }

    body {
      margin: 0;
      font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
      background: var(--bg);
      color: var(--text);
      line-height: 1.6;
    }

    a {
      color: inherit;
      text-decoration: none;
    }

    .container {
      width: min(1160px, calc(100% - 32px));
      margin: 0 auto;
    }

    .topbar {
      position: sticky;
      top: 0;
      z-index: 20;
      background: rgba(255, 255, 255, .88);
      backdrop-filter: blur(14px);
      border-bottom: 1px solid var(--border);
    }

    .nav {
      min-height: 72px;
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 24px;
    }

    .brand {
      display: flex;
      align-items: center;
      gap: 10px;
      font-size: 1.1rem;
      font-weight: 600;
      color: var(--dark);
      letter-spacing: -.03em;
    }

    .brand-icon {
      width: 38px;
      height: 38px;
      border-radius: 12px;
      display: grid;
      place-items: center;
      color: #fff;
      background: linear-gradient(135deg, var(--primary), #7c3aed);
      box-shadow: 0 10px 22px rgba(79, 70, 229, .26);
    }

    .nav-links {
      display: flex;
      align-items: center;
      gap: 22px;
      font-size: .94rem;
      font-weight: 700;
      color: #475569;
    }

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

    .nav-cta {
      padding: 10px 16px;
      border-radius: 999px;
      color: #fff !important;
      background: var(--primary);
      box-shadow: 0 10px 22px rgba(79, 70, 229, .2);
    }

    .mobile-menu-btn {
      display: none;
      width: 42px;
      height: 42px;
      border: 1px solid var(--border);
      border-radius: 12px;
      background: #fff;
      color: var(--dark);
      font-size: 1.25rem;
      cursor: pointer;
    }

    .mobile-menu {
      display: none;
      padding: 0 0 16px;
    }

    .mobile-menu.is-open {
      display: grid;
      gap: 8px;
    }

    .mobile-menu a {
      display: block;
      padding: 11px 12px;
      border-radius: 12px;
      background: #f8fafc;
      color: #475569;
      font-weight: 800;
    }

    .hero {
      position: relative;
      overflow: hidden;
      padding: 82px 0 64px;
      background:
        radial-gradient(circle at 12% 10%, rgba(79, 70, 229, .15), transparent 32%),
        radial-gradient(circle at 90% 12%, rgba(245, 158, 11, .16), transparent 28%),
        linear-gradient(180deg, #fff, #f8fafc);
    }

    .hero-grid {
      display: grid;
      grid-template-columns: minmax(0, 1.05fr) minmax(340px, .95fr);
      gap: 40px;
      align-items: center;
    }

    .eyebrow {
      display: inline-flex;
      align-items: center;
      gap: 8px;
      padding: 7px 12px;
      border: 1px solid #c7d2fe;
      background: #eef2ff;
      color: #3730a3;
      border-radius: 999px;
      font-size: .82rem;
      font-weight: 800;
      margin-bottom: 18px;
    }

    h1 {
      margin: 0 0 18px;
      font-family: Nunito;
      font-size: clamp(2.3rem, 5vw, 4.8rem);
      line-height: .98;
      letter-spacing: -.07em;
      color: var(--dark);
    }

    .hero-sub {
      max-width: 650px;
      margin: 0 0 28px;
      color: #475569;
      font-size: clamp(1.04rem, 2vw, 1.22rem);
    }

    .hero-actions {
      display: flex;
      flex-wrap: wrap;
      align-items: center;
      gap: 12px;
      margin-bottom: 28px;
    }

    .btn {
      display: inline-flex;
      align-items: center;
      justify-content: center;
      gap: 9px;
      min-height: 48px;
      padding: 13px 20px;
      border-radius: 999px;
      font-weight: 600;
      border: 1px solid transparent;
      transition: transform .16s ease, box-shadow .16s ease, background .16s ease;
      cursor: pointer;
    }

    .btn:hover {
      transform: translateY(-1px);
    }

    .btn-primary {
      background: var(--primary);
      color: #fff;
      box-shadow: 0 16px 30px rgba(79, 70, 229, .24);
    }

    .btn-soft {
      background: #fff;
      border-color: var(--border);
      color: var(--dark);
      box-shadow: 0 12px 26px rgba(15, 23, 42, .05);
    }

    .trust-row {
      display: flex;
      flex-wrap: wrap;
      gap: 10px;
    }

    .trust-pill {
      display: inline-flex;
      align-items: center;
      gap: 7px;
      padding: 8px 11px;
      border-radius: 999px;
      background: #fff;
      border: 1px solid var(--border);
      color: #475569;
      font-size: .88rem;
      font-weight: 750;
    }

    .hero-card {
      position: relative;
      padding: 22px;
      border-radius: 28px;
      background: rgba(255, 255, 255, .86);
      border: 1px solid rgba(226, 232, 240, .9);
      box-shadow: var(--shadow);
    }

    .problem-window {
      border-radius: 22px;
      overflow: hidden;
      border: 1px solid var(--border);
      background: #fff;
    }

    .window-head {
      display: flex;
      align-items: center;
      justify-content: space-between;
      padding: 14px 16px;
      background: #0f172a;
      color: #e2e8f0;
    }

    .dots {
      display: flex;
      gap: 6px;
    }

    .dot {
      width: 10px;
      height: 10px;
      border-radius: 50%;
      background: #94a3b8;
    }

    .dot:nth-child(1) { background: #fb7185; }
    .dot:nth-child(2) { background: #fbbf24; }
    .dot:nth-child(3) { background: #34d399; }

    .window-label {
      font-size: .78rem;
      font-weight: 800;
      opacity: .9;
    }

    .window-body {
      padding: 22px;
    }

    .difficulty {
      display: inline-flex;
      padding: 5px 10px;
      border-radius: 999px;
      background: #fff7ed;
      color: #c2410c;
      font-size: .76rem;
      font-weight: 600;
      margin-bottom: 12px;
    }

    .math-problem {
      margin: 0 0 16px;
      color: var(--dark);
      font-size: 1.25rem;
      font-weight: 600;
      line-height: 1.35;
    }

    .solution {
      padding: 14px;
      border-radius: 16px;
      background: #f8fafc;
      border: 1px solid var(--border);
      color: #475569;
      font-size: .95rem;
    }

    .answer-line {
      margin-top: 12px;
      padding-top: 12px;
      border-top: 1px solid var(--border);
      color: var(--dark);
      font-weight: 600;
    }

    .stats {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      gap: 14px;
      margin-top: 28px;
    }

    .stat {
      padding: 18px;
      border-radius: var(--radius);
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 10px 25px rgba(15, 23, 42, .04);
      text-align: center;
    }

    .stat strong {
      display: block;
      color: var(--dark);
      font-size: 1.55rem;
      line-height: 1;
      letter-spacing: -.04em;
    }

    .stat span {
      display: block;
      margin-top: 6px;
      color: var(--muted);
      font-size: .86rem;
      font-weight: 700;
    }

    .section {
      padding: 66px 0;
    }

    .section-head {
      max-width: 720px;
      margin: 0 auto 34px;
      text-align: center;
    }

    .section-kicker {
      margin-bottom: 8px;
      color: var(--primary);
      font-weight: 950;
      text-transform: uppercase;
      letter-spacing: .08em;
      font-size: .76rem;
    }

    h2 {
      margin: 0;
      color: var(--dark);
      font-size: clamp(1.8rem, 3vw, 2.7rem);
      line-height: 1.08;
      letter-spacing: -.055em;
    }

    .section-text {
      margin: 12px 0 0;
      color: var(--muted);
      font-size: 1.02rem;
    }

    .cards-grid {
      display: grid;
      grid-template-columns: repeat(4, minmax(0, 1fr));
      gap: 16px;
    }

    .topic-card {
      min-height: 190px;
      padding: 20px;
      border-radius: var(--radius);
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 12px 30px rgba(15, 23, 42, .045);
      display: flex;
      flex-direction: column;
      transition: transform .16s ease, box-shadow .16s ease, border-color .16s ease;
    }

    .topic-card:hover {
      transform: translateY(-3px);
      border-color: #c7d2fe;
      box-shadow: 0 18px 40px rgba(79, 70, 229, .11);
    }

    .topic-icon {
      width: 44px;
      height: 44px;
      border-radius: 15px;
      display: grid;
      place-items: center;
      background: var(--primary-light);
      color: var(--primary);
      font-size: 1.28rem;
      margin-bottom: 14px;
    }

    .topic-card h3 {
      margin: 0 0 8px;
      color: var(--dark);
      font-size: 1.05rem;
      line-height: 1.25;
      letter-spacing: -.02em;
    }

    .topic-card p {
      margin: 0 0 14px;
      color: var(--muted);
      font-size: .92rem;
    }

    .topic-meta {
      margin-top: auto;
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 12px;
      color: #475569;
      font-size: .82rem;
      font-weight: 850;
    }

    .level {
      padding: 5px 9px;
      border-radius: 999px;
      background: #f1f5f9;
    }

    .level-hard {
      background: #fff7ed;
      color: #c2410c;
    }

    .level-advanced {
      background: #fdf2f8;
      color: #be185d;
    }

    .level-college {
      background: #ecfeff;
      color: #0e7490;
    }

    .problem-layout {
      display: grid;
      grid-template-columns: minmax(0, 1fr) 330px;
      gap: 22px;
      align-items: start;
    }

    .problem-list {
      display: grid;
      gap: 16px;
    }

    .practice-card {
      padding: 22px;
      border-radius: 22px;
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 12px 30px rgba(15, 23, 42, .045);
    }

    .practice-top {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 14px;
      margin-bottom: 13px;
    }

    .tag {
      display: inline-flex;
      align-items: center;
      gap: 6px;
      padding: 5px 10px;
      border-radius: 999px;
      background: var(--primary-light);
      color: var(--primary-dark);
      font-size: .76rem;
      font-weight: 600;
    }

    .practice-card h3 {
      margin: 0 0 12px;
      color: var(--dark);
      font-size: 1.22rem;
      line-height: 1.35;
      letter-spacing: -.025em;
    }

    .steps {
      margin: 0;
      padding-left: 19px;
      color: #475569;
    }

    .steps li {
      margin: 6px 0;
    }

    .answer-box {
      margin-top: 16px;
      padding: 13px 14px;
      border-radius: 14px;
      background: #ecfdf5;
      color: #065f46;
      border: 1px solid #bbf7d0;
      font-weight: 600;
    }

    .sidebar {
      position: sticky;
      top: 92px;
      display: grid;
      gap: 16px;
    }

    .side-card {
      padding: 20px;
      border-radius: 22px;
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 12px 30px rgba(15, 23, 42, .045);
    }

    .side-card h3 {
      margin: 0 0 12px;
      color: var(--dark);
      font-size: 1.05rem;
    }

    .side-list {
      display: grid;
      gap: 9px;
      margin: 0;
      padding: 0;
      list-style: none;
    }

    .side-list a {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 12px;
      padding: 10px 12px;
      border-radius: 13px;
      background: #f8fafc;
      color: #475569;
      font-weight: 800;
      font-size: .9rem;
    }

    .side-list a:hover {
      background: var(--primary-light);
      color: var(--primary-dark);
    }

    .quiz-section {
      background: #0f172a;
      color: #e2e8f0;
    }

    .quiz-section h2,
    .quiz-section .section-kicker {
      color: #fff;
    }

    .quiz-section .section-text {
      color: #cbd5e1;
    }

    .quiz-box {
      max-width: 850px;
      margin: 0 auto;
      padding: 24px;
      border-radius: 26px;
      background: rgba(255, 255, 255, .07);
      border: 1px solid rgba(255, 255, 255, .12);
      box-shadow: 0 18px 50px rgba(0, 0, 0, .18);
    }

    .quiz-question {
      margin: 0 0 18px;
      color: #fff;
      font-size: 1.35rem;
      font-weight: 600;
      letter-spacing: -.02em;
    }

    .answers {
      display: grid;
      grid-template-columns: repeat(2, minmax(0, 1fr));
      gap: 12px;
    }

    .answer-btn {
      width: 100%;
      border: 1px solid rgba(255, 255, 255, .14);
      background: rgba(255, 255, 255, .08);
      color: #fff;
      border-radius: 16px;
      padding: 15px;
      text-align: left;
      font: inherit;
      font-weight: 800;
      cursor: pointer;
      transition: background .15s ease, border-color .15s ease, transform .15s ease;
    }

    .answer-btn:hover {
      transform: translateY(-1px);
      background: rgba(255, 255, 255, .13);
    }

    .answer-btn.correct {
      background: rgba(16, 185, 129, .25);
      border-color: rgba(16, 185, 129, .65);
    }

    .answer-btn.wrong {
      background: rgba(239, 68, 68, .25);
      border-color: rgba(239, 68, 68, .65);
    }

    .quiz-feedback {
      display: none;
      margin-top: 18px;
      padding: 16px;
      border-radius: 16px;
      background: rgba(255, 255, 255, .09);
      color: #e2e8f0;
    }

    .article-grid {
      display: grid;
      grid-template-columns: repeat(3, minmax(0, 1fr));
      gap: 16px;
    }

    .article-card {
      padding: 22px;
      border-radius: var(--radius);
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 12px 30px rgba(15, 23, 42, .045);
    }

    .article-card h3 {
      margin: 0 0 9px;
      color: var(--dark);
      font-size: 1.1rem;
      line-height: 1.28;
    }

    .article-card p {
      margin: 0 0 16px;
      color: var(--muted);
      font-size: .94rem;
    }

    .read-more {
      display: inline-flex;
      align-items: center;
      gap: 8px;
      color: var(--primary);
      font-weight: 950;
      font-size: .9rem;
    }

    .faq-grid {
      display: grid;
      gap: 14px;
      max-width: 860px;
      margin: 0 auto;
    }

    .faq-item {
      padding: 20px;
      border-radius: 18px;
      background: #fff;
      border: 1px solid var(--border);
      box-shadow: 0 10px 24px rgba(15, 23, 42, .04);
    }

    .faq-item h3 {
      margin: 0 0 8px;
      color: var(--dark);
      font-size: 1.05rem;
    }

    .faq-item p {
      margin: 0;
      color: var(--muted);
    }

    .cta {
      padding: 64px 0 76px;
    }

    .cta-box {
      overflow: hidden;
      position: relative;
      padding: 42px;
      border-radius: 30px;
      background:
        radial-gradient(circle at 15% 20%, rgba(255,255,255,.22), transparent 28%),
        linear-gradient(135deg, var(--primary), #7c3aed);
      color: #fff;
      box-shadow: 0 24px 60px rgba(79, 70, 229, .25);
      display: grid;
      grid-template-columns: 1fr auto;
      gap: 24px;
      align-items: center;
    }

    .cta-box h2 {
      color: #fff;
    }

    .cta-box p {
      margin: 12px 0 0;
      color: rgba(255, 255, 255, .84);
      max-width: 680px;
    }

    .cta .btn {
      background: #fff;
      color: var(--primary-dark);
    }

    .footer {
      padding: 30px 0;
      border-top: 1px solid var(--border);
      background: #fff;
      color: var(--muted);
      font-size: .92rem;
    }

    .footer-row {
      display: flex;
      justify-content: space-between;
      gap: 18px;
      flex-wrap: wrap;
    }

    .footer a {
      color: #475569;
      font-weight: 800;
      margin-left: 16px;
    }

    @media (max-width: 980px) {
      .hero-grid,
      .problem-layout,
      .cta-box {
        grid-template-columns: 1fr;
      }

      .sidebar {
        position: static;
      }

      .cards-grid {
        grid-template-columns: repeat(2, minmax(0, 1fr));
      }

      .article-grid {
        grid-template-columns: repeat(2, minmax(0, 1fr));
      }

      .stats {
        grid-template-columns: repeat(2, 1fr);
      }

      .nav-links {
        display: none;
      }

      .mobile-menu-btn {
        display: grid;
        place-items: center;
      }
    }

    @media (max-width: 620px) {
      .container {
        width: min(100% - 24px, 1160px);
      }

      .hero {
        padding: 54px 0 44px;
      }

      .hero-card {
        padding: 14px;
        border-radius: 22px;
      }

      .cards-grid,
      .article-grid,
      .answers {
        grid-template-columns: 1fr;
      }

      .topic-card {
        min-height: auto;
      }

      .practice-top {
        align-items: flex-start;
        flex-direction: column;
      }

      .cta-box {
        padding: 28px;
        border-radius: 24px;
      }

      .footer-row {
        display: block;
      }

      .footer a {
        display: inline-block;
        margin: 12px 16px 0 0;
      }
    }