:root{--paper:#f7f6f0;--ink:#203638;--muted:#536c70;--teal:#176c64;--blue:#245dac;--amber:#a45612;--line:#ccd8d7;--wash:#eaf2ef;--white:#fff;--center:#176c64;font-family:Segoe UI,system-ui,sans-serif;color:var(--ink);background:var(--paper);font-synthesis:none}
*{box-sizing:border-box}body{margin:0}button,select,input{font:inherit;color:inherit}button,select{border:1px solid var(--line);background:#fff;border-radius:9px;padding:11px 16px;cursor:pointer;min-height:44px}button:hover{border-color:var(--teal);background:#f0f6f3}button:disabled{opacity:.45;cursor:default}button:focus-visible,a:focus-visible,select:focus-visible,input:focus-visible,summary:focus-visible{outline:3px solid var(--amber);outline-offset:3px}.primary{background:var(--teal);border-color:var(--teal);color:#fff;font-weight:650}.primary:hover{background:#104e49;color:#fff}a{color:#17666d;text-underline-offset:3px}.skip{position:fixed;z-index:10;top:-80px;left:12px;background:#fff;padding:14px}.skip:focus{top:12px}
.site-header{padding:20px 32px 0;display:flex;flex-wrap:wrap;gap:16px;align-items:center;border-bottom:1px solid var(--line)}.brand{display:flex;align-items:center;gap:13px;margin-right:auto}.brand-mark{font-family:Cambria,serif;font-size:32px;color:var(--teal);letter-spacing:-3px}.brand strong{font-size:21px;letter-spacing:-.4px}.brand p{margin:4px 0 0;font-size:15px}.brand p span{font-size:12px;color:var(--muted);margin-left:5px;display:inline-block}.utilities{display:flex;gap:6px}.utilities button{border-color:transparent;background:transparent;font-size:13px;padding:8px 12px}nav{display:flex;gap:7px;width:100%;padding:0 0 16px;flex-wrap:wrap}nav button{background:transparent;border-color:transparent;white-space:nowrap}nav small{opacity:.65;font-size:10px;margin-right:8px}nav button[aria-current="step"]{background:var(--ink);color:#fff}
main{max-width:1450px;margin:auto;padding:26px 32px 30px}.intro{display:flex;justify-content:space-between;gap:35px;align-items:center}.intro .lead{max-width:420px;line-height:1.6;margin:0;color:var(--muted)}.eyebrow{text-transform:uppercase;letter-spacing:1.1px;font-size:11px;font-weight:650;color:var(--teal);margin:0 0 8px}h1{font-size:clamp(26px,2.6vw,36px);line-height:1.18;letter-spacing:-.7px;margin:0 0 10px}h2{font-size:21px;line-height:1.3;margin:0 0 12px}h3{font-size:16px;margin:0 0 9px}p{line-height:1.55}.choices{display:flex;flex-wrap:wrap;gap:8px;margin:18px 0}.choices [aria-pressed="true"]{background:var(--wash);border-color:var(--teal);color:#075957}.story{background:#eeefe6;border-left:3px solid #91b3a9;border-radius:0 12px 12px 0;padding:14px 18px;margin:18px 0;line-height:1.55}.story p{margin:0}.story .small{display:block;margin-top:3px}.board{display:grid;grid-template-columns:minmax(300px,.85fr) minmax(450px,1.55fr);background:#fff;border:1px solid var(--line);border-radius:19px;overflow:hidden;box-shadow:0 15px 36px #243b3510}.reasoning{padding:25px 27px;border-right:1px solid var(--line)}.geometry{padding:24px 26px 16px;min-width:0}.geometry-header{display:flex;align-items:center;justify-content:space-between;gap:10px}.tag{font-size:11px;border:1px solid var(--line);border-radius:20px;padding:6px 10px;white-space:nowrap}.math{font-family:Cambria Math,Cambria,Georgia,serif;font-size:27px;line-height:1.5;font-variant-numeric:lining-nums}.math i{font-style:italic}.hero-math{font-size:40px;margin:10px 0 20px}.math.small-math{font-size:23px}.reasoning p{margin:10px 0}.muted,.small{color:var(--muted)}.small{font-size:13px}.math-note{font-size:14px;color:var(--muted);line-height:1.5}.step-row{padding:10px 0;border-top:1px solid #e2e9e6}.step-row:first-child{border-top:0}.step-row .operation{font-family:Segoe UI,system-ui,sans-serif;font-size:13px;color:var(--teal);font-weight:650;display:block}.solution{background:#edf3fc;border-left:3px solid var(--blue);padding:12px 15px;margin-top:15px;border-radius:0 9px 9px 0}.solution .math{color:#214e8c}.branch{display:grid;grid-template-columns:1fr auto 1fr;gap:10px;align-items:center;text-align:center}.branch .math{font-size:23px}.join{font-size:11px;letter-spacing:1px;font-weight:700;color:var(--teal)}.structure{display:flex;gap:8px;flex-wrap:wrap;margin:13px 0}.structure span{font-size:12px;background:var(--wash);padding:5px 9px;border-radius:6px}.graph-scroll{overflow-x:auto;overscroll-behavior-x:contain}.graph-scroll svg{display:block;width:100%;height:auto;min-width:490px}.graph-scroll svg text{font-family:Segoe UI,system-ui,sans-serif;font-size:16px;fill:var(--muted)}.graph-scroll svg .strong{font-weight:650;fill:var(--ink)}.graph-scroll svg .blue-label{fill:var(--blue);font-weight:650}.graph-scroll svg .center-label{fill:var(--teal);font-weight:650}.axis{stroke:#879d9c;stroke-width:1.5}.center-guide{stroke:var(--teal);stroke-width:1.7;stroke-dasharray:5 5}.solution-line{stroke:var(--blue);stroke-width:7;fill:none;stroke-linecap:round}.solution-dot{stroke:var(--blue);stroke-width:3}.distance-span{stroke:var(--teal);stroke-width:2;fill:none}.probe-ring{fill:#fff;stroke:var(--amber);stroke-width:3.5}.graph-caption{font-size:13px;line-height:1.6;border-top:1px solid #e2e9e6;padding-top:12px;margin:8px 0 0}.legend{display:flex;gap:15px;flex-wrap:wrap;margin:10px 0;font-size:12px;color:var(--muted)}.legend b{font-weight:400}.sample{display:inline-block;width:19px;margin-right:5px;vertical-align:middle;border-top:3px solid var(--blue)}.sample.center{border-top:2px dashed var(--teal)}.action{display:flex;align-items:flex-start;gap:13px;background:#eef2ea;border-radius:12px;padding:15px 20px;margin:18px 0 0}.action b{display:block;font-size:11px;text-transform:uppercase;letter-spacing:1px;color:var(--teal);margin-bottom:4px}.action p{margin:0}.action .symbol{font-size:25px;color:var(--teal)}.takeaway{display:flex;align-items:center;gap:14px;font-size:17px;line-height:1.5;padding:18px 5px 0}.takeaway span{font-size:28px;color:var(--teal)}
.check-panel{background:#fff;border:1px solid var(--line);border-radius:15px;display:grid;grid-template-columns:1fr 1.2fr;gap:24px;padding:21px 24px;margin-top:18px}.quick-values{display:flex;gap:7px;flex-wrap:wrap;margin:12px 0}.quick-values button{padding:7px 12px;min-width:42px}.check-form{display:flex;gap:8px;align-items:center;flex-wrap:wrap}.check-form label{font-size:13px}.check-form input{border:1px solid #849f9e;border-radius:7px;background:white;width:92px;padding:9px}.check-result{padding:16px 20px;background:#f6f8f4;border-radius:11px;align-self:stretch}.check-result p{margin:7px 0}.check-result .math{font-size:25px}.check-result h3{color:var(--teal)}.check-result .does-not{color:#994517}.practice{margin-top:20px;border:1px solid var(--line);border-radius:12px;background:#fff;padding:15px 20px}.practice summary{font-weight:600;cursor:pointer;min-height:30px}.practice select{max-width:100%;margin-right:9px}.practice-answer{padding-top:14px}.practice-answer:empty{display:none}.center-lab{margin-top:20px;padding:18px;border:1px solid var(--line);border-radius:12px}.center-lab summary{cursor:pointer;font-weight:600}.center-lab input{width:100%;accent-color:var(--teal)}.center-lab .math{font-size:22px}.center-lab svg{min-width:490px}details p{margin:12px 0}.review{margin-top:18px}.review summary{cursor:pointer;color:var(--teal);font-size:13px} .review .math{font-size:22px}.status{font-size:13px;color:var(--amber);margin-top:8px}
.lesson-controls{position:sticky;bottom:0;z-index:2;background:#f7f6f0f5;backdrop-filter:blur(9px);border-top:1px solid var(--line);padding:17px 32px;display:flex;gap:20px;align-items:center;justify-content:space-between}.progress{min-width:170px;max-width:370px}.progress span{font-size:10px;text-transform:uppercase;letter-spacing:1px;font-weight:650;color:var(--teal)}.progress p{font-size:12px;margin:5px 0 0}.advance{display:flex;gap:9px;align-items:center}.advance .primary{min-width:210px}.motion-controls{display:flex;gap:8px;align-items:center;font-size:12px}.motion-controls button{font-size:12px}.motion-controls select{font-size:12px}.footnote{display:flex;justify-content:space-between;gap:14px;font-size:11px;color:var(--muted);padding:14px 32px 22px}.footnote div{display:flex;gap:24px}dialog{border:1px solid var(--line);border-radius:18px;background:var(--paper);color:var(--ink);padding:30px;max-width:650px;width:calc(100% - 36px);max-height:85vh;box-shadow:0 20px 80px #20363833}dialog::backdrop{background:#2036389e;backdrop-filter:blur(8px)}#paper-dialog::backdrop{background:var(--paper);backdrop-filter:none}dialog p,dialog li{font-size:15px;line-height:1.65}dialog li{margin:10px 0}dialog ol{padding-left:23px}dialog .close{float:right;padding:0;width:40px;min-height:40px;font-size:26px;border:0;background:transparent}.sr-only{position:absolute;width:1px;height:1px;padding:0;margin:-1px;overflow:hidden;clip:rect(0,0,0,0);white-space:nowrap;border:0}
@media(min-width:1500px){.site-header,.lesson-controls,.footnote{padding-left:max(32px,calc((100vw - 1386px)/2));padding-right:max(32px,calc((100vw - 1386px)/2))}}
@media(max-width:1080px){.brand p span{display:block;margin:3px 0 0}.site-header{padding:15px 22px 0}.utilities{gap:0}.utilities button{padding:7px 8px}.board{grid-template-columns:minmax(265px,.85fr) minmax(400px,1.35fr)}main{padding:22px}.reasoning,.geometry{padding:20px}.intro .lead{max-width:300px;font-size:14px}.lesson-controls{padding:12px 22px;gap:12px;flex-wrap:wrap}.progress{max-width:310px;flex:1}.motion-controls{margin-left:auto}.advance .primary{min-width:190px}nav{gap:2px}nav button{padding:9px 10px;font-size:14px}}
@media(max-width:800px){.board{grid-template-columns:1fr}.reasoning{border-right:0;border-bottom:1px solid var(--line)}.intro{display:block}.intro .lead{max-width:none;margin:4px 0 12px}.utilities{margin-left:auto}.check-panel{grid-template-columns:1fr;gap:15px}.progress{max-width:none}.motion-controls{width:100%;justify-content:flex-end}.footnote{flex-direction:column}.graph-scroll svg{min-width:490px}.reasoning .hero-math{margin:8px 0 12px}.branch .math{font-size:26px}.story{font-size:14px}.board .math{overflow-wrap:anywhere}.lesson-controls{position:static}.brand strong{font-size:20px}}
@media(max-width:480px){main{padding:18px 14px}.site-header{padding:14px 14px 0}.brand-mark{font-size:28px}.brand p{font-size:14px}.brand p span{font-size:11px}.utilities{margin:0;width:100%;justify-content:flex-end}.site-header{gap:8px}nav{gap:3px}nav button{font-size:12px;padding:7px 9px;min-height:40px}nav small{margin-right:5px}.choices button{padding:9px 12px;font-size:13px}.reasoning,.geometry{padding:18px}.geometry-header h2{font-size:18px}.tag{display:none}.graph-caption{font-size:12px}.action{padding:14px}.action p{font-size:14px}.lesson-controls{padding:14px;display:block}.advance{margin:12px 0}.advance .primary{flex:1;min-width:0}.motion-controls{justify-content:space-between}.footnote{padding:14px}.check-panel{padding:18px}.branch{gap:5px}.branch .math{font-size:21px}.math{font-size:25px}.hero-math{font-size:34px}.takeaway{font-size:15px}.practice{padding:14px}.practice select{width:100%;margin:6px 0 12px}}
@media print{.site-header,.lesson-controls,.footnote,.check-panel,.practice,.center-lab,.action,.choices{display:none}main{padding:0}.board{box-shadow:none}.graph-scroll{overflow:visible}.graph-scroll svg{min-width:0}body{background:white}.board{break-inside:avoid}}

.operations .math{font-size:22px}.operations .join{display:block;padding:2px 0}.operations{overflow-wrap:normal}@media(max-width:1080px) and (min-width:801px){.operations .math{font-size:18px}}@media(max-width:480px){.operations .math{font-size:20px}}

.probe-ring{fill:none}