:root{--ink:#111e31;--muted:#526071;--accent:#286447;--pale:#eaf3ed;--surface:#f4f5f3;--line:#d9e0e4;--ui:system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;--heading:Georgia,"Times New Roman",serif}
*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;background:#fff;color:var(--ink);font:17px/1.65 var(--ui)}a{color:var(--accent);text-underline-offset:4px}button,input,select{font:inherit}button,select,input{border:1px solid #cbd5db;border-radius:4px;background:#fff;color:var(--ink);padding:10px 15px}button{cursor:pointer;line-height:1.4}button:hover{border-color:var(--accent);background:#f3f8f4}button.primary{background:var(--accent);color:white;border-color:var(--accent)}button.primary:hover{background:#205339}button:disabled{opacity:.6;cursor:wait}:focus-visible{outline:3px solid #adcfbb;outline-offset:4px}.site-header{height:84px;border-bottom:1px solid var(--line);display:flex;align-items:center;justify-content:space-between;padding:0 48px;gap:24px}.brand{font:bold 31px/1.15 var(--heading);color:var(--ink);text-decoration:none}.site-header nav{display:flex;gap:30px;font-size:15px}.site-header nav a{color:var(--ink);text-decoration:none}.site-header nav a:hover{text-decoration:underline}.layout{display:grid;grid-template-columns:288px minmax(0,1fr);min-height:calc(100vh - 84px)}.sidebar{background:var(--surface);border-right:1px solid var(--line);padding:32px 22px 24px;display:flex;flex-direction:column;gap:16px}.sidebar h2{font:700 21px/1.4 var(--ui);margin:0 10px}.sidebar select{width:100%;font-size:15px}.sidebar details summary{display:none}.sidebar nav a{display:flex;align-items:center;gap:16px;text-decoration:none;padding:10px 12px;color:#37485b;font-size:15px;line-height:1.45;border-left:3px solid transparent;margin:3px 0;border-radius:0 4px 4px 0}.sidebar nav a span{font-size:13px;width:22px;flex-shrink:0}.sidebar nav a[aria-current=page]{border-color:var(--accent);background:#e1ede4;color:var(--accent);font-weight:600}.sidebar nav a:hover{background:#e8ece9}.sidebar nav a .done{margin-left:auto;width:auto;color:var(--accent)}.progress-block{margin-top:auto;padding:28px 10px 0}.progress-block p{margin:6px 0;color:var(--muted);font-size:14px}progress{width:100%;height:9px;border:0;border-radius:8px;overflow:hidden;background:#dce2df}progress::-webkit-progress-bar{background:#dce2df}progress::-webkit-progress-value{background:var(--accent);border-radius:8px}progress::-moz-progress-bar{background:var(--accent)}main{padding:34px 54px 58px;max-width:1220px;width:100%;margin:auto}.crumb{font-size:14px;color:var(--muted);margin-bottom:18px}.lesson-actions{display:flex;gap:16px;align-items:center;justify-content:space-between;margin:0 0 10px}.lesson-actions p{margin:0;font-size:14px;color:var(--muted)}.pin{font-size:14px;white-space:nowrap}.pin[aria-pressed=true]{background:var(--pale);border-color:var(--accent);color:var(--accent)}.markdown-document.editor.rich-editor{font:17px/1.7 var(--ui);color:var(--ink);background:transparent;border:0;padding:0;min-height:0;box-shadow:none;max-width:none;overflow:visible;--editor-text-color:var(--ink);--editor-bg-color:#fff}.markdown-document h1,main h1{font:700 clamp(30px,3.2vw,44px)/1.16 var(--heading);color:var(--ink);letter-spacing:-.8px;margin:12px 0 16px}.markdown-document h1+p{font-size:19px;color:var(--muted);margin-bottom:36px}.markdown-document h2,main h2{font:700 29px/1.25 var(--heading);letter-spacing:-.3px;margin:34px 0 14px;color:var(--ink)}.markdown-document h3,main h3{font:700 19px/1.5 var(--ui)}.markdown-document p{margin:14px 0}.markdown-document li{padding-left:8px;margin:14px 0}.markdown-document ol{padding-left:25px}.markdown-document .math-block,.markdown-document .rich-math-block{margin:22px 0;padding:22px;background:var(--pale);border:0;border-radius:4px;color:#163f2c;overflow-x:auto}.markdown-document .math-block .katex,.markdown-document .rich-math-block .katex{font-size:1.45em}.katex-display{margin:0}.markdown-document table{display:block;max-width:100%;overflow:auto;border-collapse:collapse;font-size:15px}.markdown-document td,.markdown-document th{padding:12px;border-bottom:1px solid var(--line);text-align:left}.practice{border-top:1px solid var(--line);margin-top:34px;padding-top:4px}.practice h2{margin-top:22px}.exercise{padding:18px 0;border-bottom:1px solid #e7ece9}.exercise p{margin:0 0 12px}.answer-row{display:flex;align-items:center;flex-wrap:wrap;gap:12px}.answer-row label{font-size:14px}.answer-row input{width:180px}.answer-row select{max-width:100%;min-width:180px}.text-button{border:0;padding:8px 4px;color:var(--accent);text-decoration:underline;background:transparent;font-size:15px}.feedback,.hint{font-size:15px;padding:12px 15px;margin-top:14px!important;background:#f4f5f3;border-left:3px solid #afbcb3}.feedback.good{background:var(--pale);border-color:var(--accent)}.feedback.bad{background:#fcf4ed;border-color:#a96630}.feedback:empty,.hint:empty{display:none}.lesson-bottom{display:flex;gap:18px;align-items:center;justify-content:space-between;padding-top:25px;flex-wrap:wrap}.lesson-bottom a{font-size:15px}.small{font-size:14px;color:var(--muted)}.recommendation{padding:22px 25px;background:var(--pale);margin:28px 0}.recommendation p{margin:0 0 10px}.route-list{list-style:none;padding:0;margin:30px 0}.route-list li{padding:19px 0;border-bottom:1px solid var(--line);display:flex;justify-content:space-between;gap:16px}.route-list a{font-weight:600}.route-list span{font-size:14px;color:var(--muted);white-space:nowrap}.intro{max-width:740px;font-size:19px;color:var(--muted)}.profile{display:flex;gap:20px;flex-wrap:wrap;margin:25px 0}.profile label{display:block;font-size:14px}.profile select{display:block;margin-top:8px;font-size:15px}.notice{font-size:15px;padding:16px 0;color:var(--muted);border-bottom:1px solid var(--line)}.privacy-check{display:flex;align-items:flex-start;gap:14px;margin:25px 0}.privacy-check input{width:22px;height:22px;margin-top:5px;flex-shrink:0;accent-color:var(--accent)}.privacy-actions{display:flex;gap:12px;flex-wrap:wrap;margin:24px 0}.status{font-size:14px;color:var(--muted)}footer{padding:22px 48px;border-top:1px solid var(--line);font-size:13px;color:var(--muted)}.skip{position:fixed;top:-100px;left:20px;z-index:10;background:white;padding:15px}.skip:focus{top:10px}.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}.load-error{padding:25px;background:#fcf4ed}fieldset{border:0;margin:0;padding:0}button.danger{color:#8c382c}
@media(min-width:1600px){main{padding-left:70px;padding-right:70px}}@media(max-width:1000px){.layout{grid-template-columns:238px minmax(0,1fr)}main{padding:30px}.sidebar{padding:28px 14px}.site-header{padding:0 30px}.brand{font-size:27px}.site-header nav{gap:20px}.markdown-document h1{font-size:36px}.route-list li{flex-wrap:wrap}}@media(max-width:700px){.site-header{height:auto;min-height:100px;padding:20px;align-items:flex-start;flex-direction:column;gap:15px}.brand{font-size:27px}.site-header nav{font-size:14px;gap:24px}.layout{display:block}.sidebar{padding:18px 20px;border-right:0;border-bottom:1px solid var(--line);display:grid;grid-template-columns:1fr 1fr;gap:14px}.sidebar h2{font-size:18px;margin:6px 0}.sidebar select{font-size:13px;padding:8px}.sidebar details{grid-column:1/-1}.sidebar details summary{display:list-item;font-size:14px;cursor:pointer;color:var(--accent)}.sidebar .progress-block{display:none}.sidebar nav{padding-top:10px;display:grid;grid-template-columns:1fr 1fr;gap:0 8px}.sidebar nav a{font-size:13px;gap:5px;padding:8px 5px}.sidebar nav a span{width:17px}.sidebar nav .done{display:none}main{padding:26px 20px 40px}.markdown-document.editor.rich-editor{font-size:16px}.markdown-document h1,main h1{font-size:32px;line-height:1.2}.markdown-document h1+p{font-size:17px;margin-bottom:28px}.markdown-document h2,main h2{font-size:25px}.lesson-actions{align-items:flex-start;gap:8px}.pin{font-size:12px;padding:9px}.answer-row{gap:10px}.answer-row input{width:130px}.answer-row button{font-size:14px}.profile{display:block}.profile label{margin:18px 0}.profile select{width:100%}.route-list li{gap:5px}.recommendation{padding:18px}.markdown-document table{font-size:13px}.markdown-document td,.markdown-document th{padding:8px}.markdown-document .math-block{padding:18px 10px}.markdown-document .math-block .katex{font-size:1.2em}footer{padding:20px}.privacy-actions button{font-size:14px}.lesson-bottom{display:block}.lesson-bottom a{display:block;margin-top:15px}}
main:focus{outline:none}.rich-math-block-output{overflow-x:auto}.math-inline{white-space:nowrap}.rich-table-wrap{max-width:100%;overflow:auto}.markdown-document ol{list-style:none;counter-reset:steps;padding:0}.markdown-document ol>li{position:relative;counter-increment:steps;padding:5px 0 5px 65px;min-height:54px;margin:20px 0}.markdown-document ol>li::before{content:counter(steps);position:absolute;left:0;top:4px;width:42px;height:42px;display:grid;place-items:center;border-radius:50%;background:var(--pale);color:var(--ink);font-size:21px;font-weight:600}.markdown-document ol strong{display:block}.markdown-document .rich-math-block .katex{font-size:1.65em}.markdown-document .math-inline .katex{font-size:1.08em}.markdown-document .rich-math-block+.math-note{font-size:15px}.markdown-document table{width:100%;display:table}.markdown-document th{background:var(--surface)}
.lesson-actions{flex-wrap:wrap}#guided-jump{flex-basis:100%;text-align:left}
@media(max-width:700px){.markdown-document table{display:block}.markdown-document ol>li{padding-left:48px}.markdown-document ol>li::before{width:32px;height:32px;font-size:17px}.markdown-document .rich-math-block .katex{font-size:1.25em}}
@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}}
.guided-practice>label{display:block;font-size:15px;font-weight:600;margin:18px 0 8px}.guided-practice>select{width:100%;max-width:100%;white-space:normal}.guided-counter{color:var(--accent);font-size:15px;font-weight:600;margin:24px 0 10px}.guided-practice legend{font-weight:600;font-size:19px;margin-bottom:18px;max-width:100%;overflow-wrap:anywhere}.guided-options{display:grid;gap:12px}.guided-option{display:grid;grid-template-columns:22px minmax(0,1fr);align-items:start;gap:12px;padding:16px;border:1px solid var(--line);border-radius:4px;cursor:pointer;overflow-wrap:anywhere}.guided-option:hover{background:var(--surface);border-color:var(--accent)}.guided-option input{width:20px;height:20px;margin:4px 0 0;accent-color:var(--accent);padding:0}.guided-option.chosen-good{background:var(--pale);border-color:var(--accent)}.guided-option.chosen-bad{background:#fcf4ed;border-color:#a96630}.guided-controls{display:flex;flex-wrap:wrap;gap:14px;align-items:center;margin-top:18px}.guided-controls button:disabled{cursor:default}.guided-complete{background:var(--pale);padding:16px}.guided-solution{padding-left:25px}.guided-solution li{margin:14px 0}.guided-practice h3:focus,.guided-practice legend:focus{outline:none}@media(max-width:700px){.guided-option{font-size:15px;padding:13px;gap:9px}.guided-practice legend{font-size:17px}.guided-practice>select{font-size:14px}.guided-controls{gap:10px}.guided-controls button{font-size:14px}}
/* Manipulable Gauss explanation. Each unknown keeps its color across operations. */
.gauss-lab{--lab-blue:#e7effb;--lab-blue-ink:#234c83;--lab-amber:#fff0d8;--lab-amber-ink:#79500c;--lab-violet:#f0e9fa;--lab-violet-ink:#69448b;font:16px/1.55 var(--ui);border:1px solid var(--line);border-radius:12px;padding:26px;margin:26px 0;background:#fff;max-width:100%}.gauss-lab h2{font:600 28px/1.2 var(--heading);margin:5px 0 0}.gauss-lab h3{font:600 17px/1.4 var(--ui);margin:18px 0 12px}.gauss-lab p{margin:12px 0}.lab-heading{display:flex;gap:16px;align-items:flex-start;justify-content:space-between}.lab-eyebrow{font-size:12px;text-transform:uppercase;letter-spacing:.08em;color:var(--accent);font-weight:700;margin:0!important}.lab-badge{white-space:nowrap;font-size:12px;background:var(--pale);color:var(--accent);padding:6px 10px;border-radius:20px}.lab-legend{display:flex;gap:8px;align-items:center;flex-wrap:wrap;font-size:13px;margin:20px 0}.lab-legend>span:not(:last-child){padding:2px 10px;border-radius:6px;font-weight:700}.lab-x{background:var(--lab-blue);color:var(--lab-blue-ink)}.lab-y{background:var(--lab-amber);color:var(--lab-amber-ink)}.lab-z{background:var(--lab-violet);color:var(--lab-violet-ink)}.lab-result{background:var(--pale);color:var(--accent)}.lab-rows{display:grid;gap:12px}.lab-row{border:1px solid var(--line);border-radius:9px;display:flex;gap:14px;align-items:center;padding:14px;background:#fff;transition:background .2s,border-color .2s}.lab-handle{touch-action:none;user-select:none;cursor:grab;flex-shrink:0;padding:10px 8px;background:var(--surface);font-size:14px}.lab-handle:active{cursor:grabbing}.lab-equation{display:flex;align-items:center;gap:7px;flex-wrap:wrap;font-variant-numeric:tabular-nums;min-width:0}.lab-term{display:inline-flex;gap:6px;align-items:baseline;padding:8px 10px;border-radius:6px;min-width:49px;justify-content:center}.lab-term b{font-weight:650}.lab-zero{opacity:.52;border:1px dashed currentColor}.lab-plus,.lab-equals{font-size:18px;color:var(--muted)}.lab-drop{border:2px dashed var(--accent);background:var(--pale)}.lab-controls{margin-top:24px;padding:20px;background:var(--surface);border-radius:9px;display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:15px}.lab-controls label{font-size:13px;font-weight:600;min-width:0}.lab-controls label select,.lab-controls label input{display:block;width:100%;min-width:0;font-size:15px;margin-top:6px;padding:10px}.lab-controls [hidden]{display:none!important}.lab-controls>div{grid-column:1/-1}.lab-presets,.lab-actions{display:flex;gap:8px;flex-wrap:wrap}.lab-presets button{font-size:14px;padding:7px 13px;background:#fff}.lab-actions button{font-size:14px}.lab-preview{border-top:1px solid var(--line);padding-top:10px;min-width:0}.lab-preview>p:first-child{font-size:14px}.lab-feedback{border-left:3px solid var(--accent);padding:14px;background:var(--pale);overflow-wrap:anywhere}.lab-column-changes{display:grid;grid-template-columns:repeat(4,minmax(0,1fr));gap:8px}.lab-column-changes>div{padding:12px;border-radius:7px;display:flex;flex-direction:column;gap:8px;min-width:0}.lab-column-changes span{font-size:13px;overflow-wrap:anywhere}.lab-insight{padding:16px;border:1px solid var(--line);border-radius:8px;margin:20px 0}.lab-insight p{font-size:14px;margin-bottom:0}.lab-check,.lab-history,.lab-reading{font-size:14px;border-top:1px solid var(--line);padding:16px 0}.lab-check summary,.lab-history summary,.lab-reading summary{cursor:pointer;color:var(--accent);font-weight:600}.lab-check p{overflow-wrap:anywhere}.gauss-lab .lab-history ol{list-style:decimal;padding-left:24px;counter-reset:none}.gauss-lab .lab-history ol>li{position:static;padding:4px 0;min-height:0;margin:4px 0}.gauss-lab .lab-history ol>li::before{content:none}.lab-updated{animation:lab-change .6s ease-out}@keyframes lab-change{from{background:#d8eedf;transform:translateY(-3px)}to{background:white;transform:translateY(0)}}@media(max-width:1050px){.lab-heading{display:block}.lab-badge{display:inline-block;margin-top:12px}.lab-row{gap:8px;padding:10px}.lab-term{min-width:40px;padding:7px}.lab-equation{gap:5px}}@media(max-width:700px){.gauss-lab{padding:16px;margin:24px -4px}.gauss-lab h2{font-size:25px}.lab-controls{padding:14px;grid-template-columns:minmax(0,1fr)}.lab-row{display:block}.lab-handle{margin-bottom:10px}.lab-equation{gap:5px;font-size:14px}.lab-term{padding:7px;min-width:38px;gap:3px}.lab-column-changes{grid-template-columns:repeat(2,minmax(0,1fr))}.lab-heading .lab-badge{margin-top:12px}.lab-actions button{flex:1 1 auto}.lab-legend>span:last-child{flex-basis:100%}}@media(prefers-reduced-motion:reduce){.lab-updated{animation:none}.lab-row{transition:none}}