*{box-sizing:border-box} :root{--ink:#17283b;--muted:#50667e;--blue:#2463a8;--pink:#b85579;--line:#dbe4ee;--paper:#fff;--bg:#edf2f8}body{margin:0;background:var(--bg);color:var(--ink);font:17px/1.5 Arial,sans-serif}button,select{font:inherit;color:inherit}button,a,select{-webkit-tap-highlight-color:transparent}button,select{border:1px solid #c9d7e5;border-radius:10px;background:#fff;padding:9px 13px;cursor:pointer}button:hover,a.tab:hover{background:#eaf2fc}button:disabled{opacity:.4;cursor:default}button:focus-visible,a:focus-visible,select:focus-visible{outline:3px solid #388cd4;outline-offset:3px}a{color:inherit;text-decoration:none}.top{display:flex;align-items:center;gap:22px;padding:12px 24px;background:#fff;border-bottom:1px solid var(--line)}.brand{font-size:25px;font-weight:800;letter-spacing:-1px}.top nav{display:flex;gap:10px;align-items:center;flex:1;flex-wrap:wrap}.top nav a{font-size:14px;padding:7px 11px;border-radius:8px}.top nav a.active{background:#e7f0fb;color:var(--blue)}.full{font-size:13px}.chapterbar{max-width:1400px;margin:0 auto;padding:14px 24px 0;display:flex;align-items:center;justify-content:space-between;gap:16px}.chapterbar h1{font-size:20px;margin:0}.page-tabs{display:flex;padding:4px;background:#dfe8f2;border-radius:13px;gap:3px}.tab{padding:8px 18px;border-radius:10px;font-size:15px}.tab.active{background:#fff;color:var(--blue);font-weight:bold;box-shadow:0 2px 5px #1b395610}.lesson-shell{max-width:1400px;padding:12px 24px 18px;margin:auto}.topic-strip{display:flex;gap:7px;flex-wrap:wrap;margin-bottom:12px}.topic-strip button{font-size:14px;padding:8px 12px}.topic-strip button.active,.primary{background:var(--blue);color:white;border-color:var(--blue)}.workspace{display:grid;grid-template-columns:minmax(275px,.75fr) minmax(0,1.5fr);gap:16px}.card{background:#fff;border:1px solid var(--line);border-radius:20px;padding:23px;min-width:0}.source{font-size:12px;color:var(--muted);margin:0 0 8px}.eyebrow{font-size:12px;font-weight:bold;letter-spacing:1.5px;color:var(--blue);text-transform:uppercase;margin-bottom:8px}h2{font-size:25px;line-height:1.25;margin:0 0 14px}h3{font-size:18px;margin:0 0 8px}.prompt{font-size:18px;line-height:1.7}.note{margin-top:22px;padding:15px;background:#eef5ff;border-radius:12px;border-left:4px solid #74a9d9}.note p{font-size:16px;line-height:1.5;margin:5px 0}.stage{display:flex;flex-direction:column;justify-content:center;min-height:365px;position:relative}.stage .lead{font-size:14px;color:var(--muted);text-align:center;margin:0 0 14px}.formula{font-family:Georgia,'Times New Roman',serif;font-size:clamp(22px,2.3vw,34px);line-height:1.9;text-align:center;overflow-wrap:anywhere}.formula.small{font-size:23px}.formula sup,.prompt sup{font-size:.65em;line-height:0}.formula sub{font-size:.65em}.binom{display:inline-flex;vertical-align:middle;align-items:center;margin:0 .15em;line-height:1}.binom .bracket{font-size:2.2em;font-weight:300}.binom .stack{display:flex;flex-direction:column;font-size:.7em;text-align:center;gap:3px}.muted{color:var(--muted)}.blue{color:var(--blue)}.pink{color:var(--pink)}.pill{display:inline-block;border-radius:10px;background:#edf4fd;padding:6px 14px;margin:4px}.pill.b{background:#fce9ef;color:#9d365c}.reveal{animation:rise .65s ease both}.controls{margin-top:12px;background:#fff;border:1px solid var(--line);border-radius:14px;padding:12px 16px;display:flex;align-items:center;gap:8px;flex-wrap:wrap}.controls .count{margin-left:auto;color:var(--muted);font-size:14px}.controls label{font-size:13px;margin-left:4px}.controls select{font-size:14px;padding:8px}.progress{height:4px;background:#dce6f1;border-radius:5px;margin-top:10px;overflow:hidden}.progress>span{display:block;background:#70a3d1;height:100%;transition:width .35s}.keyboard{text-align:center;font-size:12px;color:var(--muted);margin:8px 0 0}.pickline{display:flex;gap:10px;align-items:center;margin-bottom:12px}.pickline select{width:100%;min-width:0;font-size:15px}.pickline label{font-size:14px;white-space:nowrap}.part-strip{display:flex;gap:6px;margin-bottom:10px;flex-wrap:wrap}.part-strip button{font-size:13px;padding:6px 12px}.part-strip .active{background:#e3eefb;border-color:#84a9d2}.exercise-stage{min-height:350px}.equation-current{padding:30px 12px;background:#f3f7fc;border-radius:15px;border:1px solid #dce8f5}.equation-current.answer{background:#eaf6ef;border-color:#acd4ba}.previous{font-size:20px;opacity:.65;text-align:center;margin:0 0 16px;min-height:40px}.start-prompt{text-align:center;padding:30px}.start-prompt .bubble{font-size:55px;color:#82a7c8}.start-prompt p{color:var(--muted);font-size:17px}.work-caption{text-align:center;font-size:14px;color:var(--muted);margin-top:18px}.people{display:flex;gap:14px;justify-content:center;margin:16px 0}.person{width:50px;height:50px;border-radius:50%;background:#d9e9ff;display:grid;place-items:center;font-weight:bold;position:relative;box-shadow:0 4px 0 #b9d0ec;color:#244b78}.person:nth-child(2){background:#ffe6b9;box-shadow:0 4px 0 #e9c786}.person:nth-child(3){background:#f8d6e0;box-shadow:0 4px 0 #e8aabb}.person:nth-child(4){background:#d7eee1;box-shadow:0 4px 0 #aed5be}.person:nth-child(5){background:#e9ddf6;box-shadow:0 4px 0 #cebae4}.seatrow{display:flex;justify-content:center;gap:12px;margin:20px 0}.seat{width:74px;height:84px;border:2px dashed #b4c7dc;border-radius:12px;display:flex;align-items:center;justify-content:center;flex-direction:column;gap:6px}.seat .person{width:39px;height:39px;animation:seat-in 1s ease both}.seat .choices{font-size:22px;font-weight:bold;color:#2560a0}.seat small{font-size:11px}.seat.current{border-color:#edae58;background:#fff5e6}.permutations{display:grid;grid-template-columns:repeat(6,1fr);gap:5px;max-width:570px;margin:12px auto}.permutations span{padding:5px;border-radius:6px;background:#eaf2fc;font-size:12px;text-align:center;animation:rise .5s both;animation-delay:calc(var(--i)*30ms)}.pairs{display:grid;grid-template-columns:repeat(5,1fr);gap:8px;margin:20px auto;max-width:570px;width:100%}.pair{background:#eaf2fc;border-radius:10px;padding:10px 3px;text-align:center;animation:rise .6s both;animation-delay:calc(var(--i)*60ms)}.pair .reverse{display:block;color:#899bb1;font-size:13px}.pair.merged{background:#e6f4ec;animation:merge 1s both}.square{width:min(100%,390px);margin:0 auto;display:block;overflow:visible}.square rect.region{animation:tile-in 1s ease both;transform-box:fill-box;transform-origin:center}.square text{font:20px Georgia,serif;fill:#203b54}.square .axistext{font:18px Arial,sans-serif}.path-stage{height:285px;position:relative;margin:6px auto;width:100%;max-width:600px}.path-chip{position:absolute;display:flex;gap:3px;align-items:center;justify-content:center;background:#f0f5fc;border:1px solid #d8e6f4;border-radius:9px;width:100px;height:34px;left:var(--x);top:var(--y);animation:move-path 1.2s ease both}.path-chip span{border-radius:5px;padding:0 4px;background:#deebfb;color:var(--blue)}.path-chip span.b{background:#f7dae5;color:#9c315a}.group-label{position:absolute;top:250px;width:25%;text-align:center;font:20px Georgia,serif}.triangle{width:100%;max-width:540px;margin:auto;display:block}.triangle text{font:20px Arial,sans-serif;fill:#254868}.triangle circle{fill:#e9f2fc;stroke:#d1e0f0}.triangle .hot circle{fill:#ffe7bb;stroke:#d59c3a;animation:pulse 1.7s ease 2}.triangle .new circle{fill:#e3f2e9;animation:rise .8s both}.triangle path{fill:none;stroke:#bc8240;stroke-width:3;stroke-dasharray:100;animation:draw 1.3s ease both}.factors{text-align:center;margin:12px auto;font-size:24px}.factor{display:inline-block;border-radius:10px;border:1px solid #d9e3ee;background:#f7faff;padding:10px 12px;margin:3px}.factor.selected{background:#f8dfe9;border-color:#d698b0;animation:flip .8s both}.terms{display:flex;gap:8px;justify-content:center;flex-wrap:wrap;margin:18px 0}.term{background:#edf4fc;border-radius:9px;padding:9px 12px;font:21px Georgia,serif}.term.current{background:#fbe4ee;animation:rise .8s both}.caption{text-align:center;font-size:15px;color:var(--muted);margin:12px 0}.home{max-width:940px;margin:50px auto;padding:0 22px}.home h1{font-size:54px;letter-spacing:-2px}.home-cards{display:grid;grid-template-columns:1fr 1fr;gap:24px}.home-card{border-radius:25px;background:white;border:1px solid var(--line);padding:30px;text-align:center;transition:transform .2s}.home-card:hover{transform:translateY(-4px)}.home-card svg{height:170px;width:100%;display:block}.home-card h2{font-size:18px;margin:18px 0 2px}.home-card p{font-size:13px;color:var(--muted)}.empty{max-width:700px;margin:60px auto;padding:35px;background:#fff;border-radius:20px}.checkpoint{background:#fff6e6;padding:10px 16px;border-radius:12px;text-align:center;margin:12px 0;font-size:16px}
@keyframes rise{from{opacity:0;transform:translateY(12px)}to{opacity:1;transform:translateY(0)}}@keyframes seat-in{from{transform:translateY(-45px);opacity:0}to{transform:translateY(0);opacity:1}}@keyframes tile-in{from{opacity:.15;transform:scale(.85)}to{opacity:1;transform:scale(1)}}@keyframes move-path{from{left:var(--from-x);top:var(--from-y)}to{left:var(--x);top:var(--y)}}@keyframes merge{from{transform:scale(1.07);background:#ffeac5}to{transform:scale(1);background:#e6f4ec}}@keyframes flip{from{transform:rotateY(90deg)}to{transform:rotateY(0)}}@keyframes pulse{50%{fill:#ffd38a}}@keyframes draw{from{stroke-dashoffset:100}to{stroke-dashoffset:0}}@media(max-width:1050px){.workspace{grid-template-columns:minmax(255px,.8fr) minmax(0,1.2fr)}.card{padding:18px}.formula{font-size:25px}.stage{min-height:340px}.top{gap:10px}.top nav{gap:0}}@media(max-width:760px){.top{padding:10px 12px;flex-wrap:wrap}.top nav{order:3;width:100%;flex-basis:100%}.top .full{margin-left:auto}.chapterbar{padding:12px;flex-wrap:wrap}.chapterbar h1{font-size:18px}.lesson-shell{padding:0 12px 14px}.workspace{grid-template-columns:1fr;gap:10px}.card{padding:16px}.stage,.exercise-stage{min-height:310px}.prompt{font-size:17px}.note{margin-top:12px}.formula{font-size:24px}.formula.small{font-size:21px}.controls{padding:10px;gap:6px}.controls button{padding:8px 11px}.count{flex-basis:100%;text-align:center}.topic-strip button{font-size:12px;padding:7px 9px}.home-cards{grid-template-columns:1fr}.home{margin-top:25px}.home h1{font-size:43px}.path-chip{width:67px;font-size:13px}.path-stage{height:285px}.seat{width:58px}.people{gap:10px}.pairs{gap:5px}.pair{font-size:14px}.previous{font-size:17px}.equation-current{padding:22px 8px}.keyboard{display:none}.page-tabs{font-size:14px}.tab{padding:7px 15px}.pickline{display:block}.pickline label{display:block;margin:8px 0 5px}.person{width:40px;height:40px}.permutations{grid-template-columns:repeat(4,1fr)}}@media(prefers-reduced-motion:reduce){*,*::before,*::after{animation:none!important;transition:none!important;scroll-behavior:auto!important}}

.path-chip{width:calc(25% - 6px)}.seat{height:98px}.seat .person{flex-shrink:0}.seat small{white-space:nowrap}

/* Do Now: reveal a single contribution at a time. */
.dn-terms{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:12px;margin-bottom:22px}.dn-term{padding:12px;border-radius:13px;background:#edf4fc;border:2px solid transparent;min-height:95px}.dn-term.current{border-color:#cb9344;background:#fff3dd;animation:rise .8s both}.dn-degree{font-size:12px;color:#50667e;display:block;text-align:center}.dn-operation{padding:20px 10px;background:#f7f9fc;border-radius:14px}.dn-keys{display:flex;justify-content:center;gap:12px;flex-wrap:wrap;margin:20px 0}.dn-keys span{padding:12px 18px;background:#edf4fc;border-radius:12px;text-align:center;animation:rise .8s both}.dn-arrow{text-align:center;color:#4276a6;font-size:23px;margin:12px}.dn-round{display:flex;justify-content:center;align-items:center;font:48px Georgia,serif;margin:22px}.dn-round strong{color:#915c0a;background:#ffebc4;border-radius:9px;padding:6px 3px;animation:rise .8s both}.page-tabs{flex-wrap:wrap}.prompt p{margin:6px 0 12px}@media(max-width:760px){.dn-terms{gap:8px}.dn-term{padding:9px 6px}.dn-term .formula{font-size:20px}.tab{padding:7px 10px}}