:root{color-scheme:light;--ink:#102b58;--muted:#647b9b;--blue:#1769e0;--line:#dce7f6;--surface:#fff;--bg:#f4f7fc}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0;color:var(--ink);background:var(--bg);font-family:Inter,"Segoe UI",Arial,sans-serif;font-size:16px;line-height:1.7}
a{color:var(--blue);text-underline-offset:3px}
a:focus-visible,summary:focus-visible{outline:3px solid #76a7ff;outline-offset:3px}
.skip-link{position:absolute;top:-70px;left:12px;z-index:10;background:#fff;padding:10px}
.skip-link:focus{top:12px}
.site-header{background:#fff;border-bottom:1px solid var(--line)}
.header-inner{max-width:1250px;margin:auto;padding:18px 32px;display:flex;align-items:center;justify-content:space-between;gap:20px}
.brand{text-decoration:none;color:var(--ink);font-size:20px;font-weight:700;white-space:nowrap;letter-spacing:-.03em}
.brand strong{color:var(--blue)}
.brand span{display:inline-grid;place-items:center;margin-right:9px;color:white;background:var(--blue);border-radius:12px;width:38px;height:38px;font-size:22px}
.home-link{font-size:14px;font-weight:650;text-decoration:none}
.page{max-width:1250px;margin:auto;padding:34px 32px 72px}
.breadcrumbs{display:flex;gap:9px;align-items:center;margin-bottom:22px;color:var(--muted);font-size:13px}
.breadcrumbs a{color:var(--muted)}
.hero{padding:40px 46px 42px;border-radius:24px;border:1px solid #d8e6fa;background:linear-gradient(120deg,#e6f0ff 0%,#f6faff 72%,#eff8e9 100%)}
.pill{display:inline-block;padding:5px 11px;border-radius:999px;background:#fff;color:var(--blue);font-size:11px;line-height:1.4;font-weight:800;letter-spacing:.06em;text-transform:uppercase}
h1,h2,h3,p{margin-top:0}
h1{max-width:720px;margin:15px 0 12px;font-size:clamp(30px,4vw,46px);line-height:1.12;letter-spacing:-.045em}
.hero p{max-width:770px;margin:0;color:#506989;font-size:16px}
.hero-note{display:inline-block;margin-top:18px;font-size:12px;font-weight:700;color:#3e6a9f}
.subject-nav{display:flex;gap:8px;flex-wrap:wrap;margin:24px 0 26px}
.subject-nav a{background:#fff;border:1px solid var(--line);border-radius:999px;color:#426080;padding:8px 15px;text-decoration:none;font-size:13px;font-weight:700}
.subject-nav a:hover,.subject-nav a[aria-current=page]{color:var(--blue);background:#eaf2ff;border-color:#b9d3ff}
.layout{display:grid;grid-template-columns:230px minmax(0,1fr);gap:28px;align-items:start}
.toc{position:sticky;top:24px;padding:18px 4px;color:var(--muted);font-size:13px}
.toc-label{font-size:10px;letter-spacing:.12em;font-weight:800;color:#8397b4}
.toc h2{font-size:17px;margin:7px 0 12px;letter-spacing:-.02em;color:var(--ink)}
.toc a{display:block;padding:7px 0;color:#536e93;text-decoration:none;font-weight:600;line-height:1.45}
.toc a:hover{color:var(--blue)}
.toc p{margin-top:22px;padding-top:16px;border-top:1px solid var(--line);line-height:1.6}
.lessons{min-width:0}
.lesson,.sources{margin-bottom:18px;padding:36px 40px;background:var(--surface);border:1px solid var(--line);border-radius:20px;box-shadow:0 3px 12px rgba(20,62,120,.025)}
.lesson{scroll-margin-top:20px}
.lesson-kicker{color:var(--blue);font-size:11px;font-weight:800;letter-spacing:.09em}
.lesson h2,.sources h2{margin:7px 0 18px;font-size:26px;line-height:1.2;letter-spacing:-.03em}
.lesson h3{margin:0 0 7px;font-size:16px;line-height:1.4}
.lesson p{margin:0 0 15px;color:#3b5476}
.lesson ol{margin:8px 0 0;padding-left:22px;color:#3b5476}
.lesson li+li{margin-top:6px}
.formula{margin:19px 0;padding:16px 20px;border-radius:12px;border-left:4px solid var(--blue);background:#edf4ff;color:#173f78;font-weight:700;overflow-wrap:anywhere}
.worked{margin:22px 0;padding:20px 22px;border:1px solid #d7e6fb;border-radius:14px;background:#f8fbff}
.worked h3{color:#135cbd}
.worked p:last-child,.worked ol:last-child{margin-bottom:0}
.practice{margin-top:22px;padding:20px 22px;border-radius:14px;background:#f1f8ec;border:1px solid #dfeeda}
.practice h3{color:#365c31}
.practice p{margin-bottom:9px}
.practice details{margin-top:12px}
.practice summary{cursor:pointer;color:#286326;font-weight:700}
.practice details p{margin:10px 0 0}
.back-top{display:inline-block;margin-top:22px;font-size:13px;font-weight:700;text-decoration:none}
.sources{background:#fbfcff}
.sources p,.sources li{color:var(--muted);font-size:14px}
.sources ul{padding-left:20px;margin-bottom:0}
.sources li+li{margin-top:6px}
.closing{display:flex;align-items:center;justify-content:space-between;gap:20px;padding:28px 30px;border-radius:18px;background:#e9f2ff;border:1px solid #d5e6ff}
.closing h2{font-size:18px;margin-bottom:5px}
.closing p{margin:0;color:#617a9b;font-size:14px}
.closing a{flex-shrink:0;color:#fff;background:var(--blue);border-radius:10px;padding:10px 15px;text-decoration:none;font-weight:700;font-size:13px}
footer{display:flex;justify-content:space-between;align-items:center;gap:18px;max-width:1250px;margin:auto;padding:28px 32px 40px;border-top:1px solid var(--line)}
footer p,footer>a{margin:8px 0 0;color:var(--muted);font-size:12px}
@media(max-width:800px){.layout{grid-template-columns:1fr;gap:4px}.toc{position:static;padding:0 4px 12px}.toc p{margin:12px 0}.page{padding:24px 20px 52px}.header-inner{padding:15px 20px}.hero{padding:30px 26px}.lesson,.sources{padding:28px}.closing{align-items:flex-start;flex-direction:column}.toc a{display:inline-block;margin-right:16px}}
@media(max-width:480px){body{font-size:15px}.header-inner{gap:10px}.brand{font-size:18px}.brand span{width:34px;height:34px;border-radius:10px}.home-link{font-size:12px}.page{padding:20px 14px 45px}.hero{padding:26px 20px;border-radius:18px}.hero p{font-size:15px}.subject-nav{gap:6px}.subject-nav a{padding:7px 11px;font-size:12px}.lesson,.sources{padding:23px 20px;border-radius:16px}.lesson h2,.sources h2{font-size:22px}.worked,.practice{padding:16px}.formula{padding:13px}.closing{padding:22px}footer{padding:25px 20px;align-items:flex-start;flex-direction:column}}
@media print{.site-header,.subject-nav,.toc,.back-top,.closing,footer,.skip-link{display:none}body{background:white}.page{padding:0}.layout{display:block}.lesson,.sources{break-inside:avoid;border:0;box-shadow:none;padding:16px 0}.hero{padding:15px 0;border:0;background:white}}