:root {
  --ink: #1b2130; --muted: #5d6677; --line: #dde1e8; --bg: #f3f4f7;
  --accent: #2456c9; --accent-soft: #eaf0fd;
  --hot: #d6452f; --hot-soft: #fdeeea;
  --ok: #17803d; --ok-soft: #e7f6ec;
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Inter, Roboto, Arial, sans-serif;
  --math: "Cambria Math", "STIX Two Math", Georgia, serif;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 64px; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 15px/1.5 var(--font); -webkit-print-color-adjust: exact; print-color-adjust: exact; }
a { color: var(--accent); }
b { font-weight: 650; }
.s { font-size: 13px; }
.muted { color: var(--muted); }

.bar { position: sticky; top: 0; z-index: 5; display: flex; gap: 12px; align-items: center; flex-wrap: wrap;
  padding: 10px 16px; background: rgba(255,255,255,.94); backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); }
.bar nav { display: flex; gap: 2px; flex-wrap: wrap; margin-right: auto; }
.bar nav a { color: var(--muted); text-decoration: none; font-size: 13px; padding: 4px 9px; border-radius: 999px; }
.bar nav a:hover { background: var(--accent-soft); color: var(--accent); }
button { font: inherit; font-size: 13px; cursor: pointer; border: 1px solid var(--line); background: #fff; color: var(--ink); padding: 7px 13px; border-radius: 8px; }
button:hover { border-color: var(--accent); color: var(--accent); }
button.primary { background: var(--accent); border-color: var(--accent); color: #fff; }
button.primary:hover { color: #fff; filter: brightness(1.08); }

main { max-width: 960px; margin: 0 auto; padding: 16px 16px 40px; }
section { background: #fff; border-radius: 14px; padding: 22px 26px; margin: 16px 0; box-shadow: 0 1px 3px rgba(0,0,0,.05); }
h1 { margin: 0 0 4px; font-size: 30px; }
h2 { display: flex; align-items: center; gap: 10px; margin: 0 0 14px; font-size: 20px; }
h2 span { display: inline-grid; place-items: center; width: 28px; height: 28px; border-radius: 8px; background: var(--accent); color: #fff; font-size: 15px; }
.lead { color: var(--muted); margin: 0 0 8px; }
.hero { background: linear-gradient(135deg, #1f3f94, #2456c9); color: #fff; }
.hero .lead { color: #d6e0fa; }
.hero a { color: #fff; }
.plan { margin: 0; padding-left: 20px; }
.plan li { margin: 4px 0; }
.plan b { display: inline-block; min-width: 56px; background: rgba(255,255,255,.18); border-radius: 6px; padding: 0 6px; margin-right: 6px; font-size: 13px; }

.grid2 { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; margin-top: 14px; }
.grid3 { display: grid; grid-template-columns: repeat(3, 1fr); gap: 14px; }
.row { display: flex; gap: 8px; align-items: center; flex-wrap: wrap; margin-bottom: 12px; }

.formulas { display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; }
.f { border: 2px solid var(--accent); background: var(--accent-soft); border-radius: 10px; padding: 10px 14px; break-inside: avoid; }
.f .eq { font-family: var(--math); font-size: 24px; }
.f p { margin: 2px 0 0; font-size: 12.5px; color: var(--muted); }
.f.hot { border-color: var(--hot); background: var(--hot-soft); }

.t { width: 100%; border-collapse: collapse; font-size: 13.5px; }
.t th, .t td { border: 1px solid var(--line); padding: 6px 9px; text-align: left; vertical-align: top; }
.t th { background: #f5f6f9; }
.box { border: 1px solid var(--line); border-radius: 10px; padding: 12px 14px; margin-top: 12px; background: #fafbfc; break-inside: avoid; }
.grid2 .box { margin-top: 0; }
.speeds div { display: grid; grid-template-columns: 62px 1fr 72px; gap: 8px; align-items: center; font-size: 13px; margin: 5px 0; }
.speeds i { display: block; height: 10px; border-radius: 5px; background: var(--accent); }
.speeds b { text-align: right; font-variant-numeric: tabular-nums; }

figure { margin: 0; break-inside: avoid; }
figure svg { width: 100%; height: auto; display: block; }
figcaption { font-size: 12.5px; color: var(--muted); text-align: center; margin-top: 4px; }
svg text { font-family: var(--font); font-size: 12px; fill: var(--ink); }
svg .sm { font-size: 10.5px; fill: var(--muted); }
svg .acc { fill: var(--accent); font-weight: 650; }
svg .b { font-weight: 700; }
svg .ax { stroke: var(--ink); stroke-width: 1.3; fill: none; }
svg .g { stroke: var(--muted); stroke-width: 1; stroke-dasharray: 4 4; fill: none; }
svg .w { stroke: var(--accent); stroke-width: 2.4; fill: none; }
svg .ar { stroke: var(--ink); stroke-width: 1.5; }
svg .ar.a { stroke: var(--accent); }

/* flip cards */
.cards { display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; }
.card { perspective: 900px; min-height: 118px; cursor: pointer; break-inside: avoid; }
.card .in { position: relative; width: 100%; height: 100%; min-height: 118px; transition: transform .45s; transform-style: preserve-3d; }
.card.hidden .in { transform: rotateY(180deg); }
.card .fr, .card .bk { position: absolute; inset: 0; border-radius: 10px; padding: 10px 12px; backface-visibility: hidden; -webkit-backface-visibility: hidden; font-size: 13.5px; }
.card .fr { background: #fff; border: 1px solid var(--line); }
.card .fr .term { font-weight: 700; color: var(--accent); display: block; margin-bottom: 3px; }
.card .bk { background: var(--accent); color: #fff; transform: rotateY(180deg); display: grid; place-items: center; text-align: center; font-weight: 700; font-size: 17px; }

/* traps */
.traps { display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; }
.trap { border-left: 4px solid var(--hot); background: var(--hot-soft); border-radius: 0 10px 10px 0; padding: 9px 12px; break-inside: avoid; }
.trap b:first-child { color: var(--hot); }
.trap p { margin: 3px 0 0; font-size: 13px; }

.mn { font-size: 17px; margin: 0 0 10px; }
.mn u { text-decoration: none; color: var(--accent); font-weight: 800; }
.t.em td:first-child { white-space: nowrap; }

.method { border: 1px solid var(--line); border-radius: 10px; padding: 10px 12px; break-inside: avoid; }
.method > b { color: var(--accent); }
.method ol { margin: 6px 0 0; padding-left: 18px; font-size: 13px; }

/* quiz */
.score { font-weight: 700; background: var(--accent-soft); color: var(--accent); padding: 5px 12px; border-radius: 8px; }
.q { border: 1px solid var(--line); border-radius: 10px; padding: 12px 14px; margin: 10px 0; }
.q .qt { font-weight: 600; margin-bottom: 8px; }
.q .qt small { color: var(--muted); font-weight: 500; margin-right: 6px; }
.opts { display: grid; grid-template-columns: 1fr 1fr; gap: 6px; }
.opts button { text-align: left; padding: 8px 12px; font-size: 14px; }
.opts button.right { background: var(--ok-soft); border-color: var(--ok); color: var(--ok); font-weight: 650; }
.opts button.wrong { background: var(--hot-soft); border-color: var(--hot); color: var(--hot); }
.opts button:disabled { cursor: default; }
.why { margin-top: 8px; font-size: 13px; padding: 7px 10px; border-radius: 8px; background: #f5f6f9; display: none; }
.q.done .why { display: block; }
.done-banner { text-align: center; padding: 18px; border-radius: 10px; background: var(--ok-soft); color: var(--ok); font-weight: 700; font-size: 17px; }

footer { text-align: center; padding: 0 16px 40px; }

@media screen and (max-width: 760px) {
  .grid2, .grid3, .formulas, .traps, .cards, .opts { grid-template-columns: 1fr; }
  .bar nav { display: none; }
  section { padding: 18px 16px; }
  h1 { font-size: 24px; }
}

/* ---------- print: dense cheat sheet ---------- */
@page { size: A4; margin: 10mm; }
@media print {
  body { background: #fff; font-size: 9.6pt; line-height: 1.35; }
  .bar, .no-print, footer, .hero { display: none !important; }
  main { max-width: none; padding: 0; }
  section { box-shadow: none; padding: 0; margin: 0 0 10px; border-radius: 0; }
  h2 { font-size: 13pt; margin-bottom: 6px; break-after: avoid; }
  h2 span { width: 20px; height: 20px; font-size: 10pt; }
  .f .eq { font-size: 15pt; }
  .card, .card .in { min-height: 0; }
  .card .in { transform: none !important; }
  .card .fr { position: static; padding: 5px 8px; }
  .card .bk { display: none; }
  .cards, .traps { gap: 6px; }
  .t { font-size: 8.8pt; }
  .t th, .t td { padding: 3px 6px; }
  a { color: inherit; text-decoration: none; }
}