:root{--paper:#f6f5ef;--panel:#fff;--ink:#203535;--muted:#536866;--line:#cdd7d2;--teal:#096b65;--wash:#e8f3ef;--rust:#a44329;--gold:#856023;--shadow:0 12px 40px #244b4210;--math:Georgia,'Times New Roman',serif}
*{box-sizing:border-box}body{margin:0;background:var(--paper);color:var(--ink);font:17px/1.45 'Segoe UI',Arial,sans-serif}button,select,input{font:inherit}button,select{min-height:44px;border:1px solid var(--line);border-radius:9px;background:#fff;color:var(--ink);padding:9px 16px;cursor:pointer}button{transition:background .18s,border-color .18s,box-shadow .18s}button:hover:not(:disabled){border-color:var(--teal);background:var(--wash)}button:disabled{opacity:.42;cursor:default}button:focus-visible,select:focus-visible,input:focus-visible,a:focus-visible{outline:3px solid #bd742c;outline-offset:4px}button.primary{background:var(--teal);border-color:var(--teal);color:#fff;font-weight:650}button.primary:hover{background:#07534e}button.quiet{background:transparent;border-color:transparent}button.active,button[aria-pressed=true]{background:var(--wash);border-color:var(--teal);color:var(--teal)}a{color:var(--teal);text-underline-offset:4px}h1,h2,p{margin:0}h1{font-size:clamp(28px,2.7vw,42px);line-height:1.13;letter-spacing:-.035em;font-weight:650}h2{font-size:26px;line-height:1.2;letter-spacing:-.02em}.masthead,.lesson-nav,main,.controls,.folio{max-width:1480px;margin-inline:auto}.masthead{display:flex;align-items:center;justify-content:space-between;padding:20px 40px 12px;gap:20px}.brand{display:flex;align-items:center;gap:12px}.brand svg{width:43px;stroke:var(--teal);stroke-width:2.5;fill:var(--teal)}.brand strong{display:block;font-size:20px;font-weight:650;letter-spacing:-.025em}.brand small{font-size:12px;letter-spacing:.075em;text-transform:uppercase;color:var(--muted)}.utilities{display:flex;gap:2px;font-size:14px}.lesson-nav{display:flex;gap:5px;padding:8px 40px 17px;border-bottom:1px solid var(--line)}.lesson-nav button{border:0;background:transparent;font-size:15px;color:var(--muted);position:relative}.lesson-nav button[aria-current=page]{background:var(--ink);color:white}.lesson-nav span{font-size:11px;opacity:.7;margin-right:9px;font-variant-numeric:tabular-nums}main{padding:24px 40px 20px;min-height:420px}.intro{display:flex;justify-content:space-between;align-items:end;gap:24px;margin-bottom:22px}.eyebrow{font-size:11px;font-weight:700;letter-spacing:.13em;text-transform:uppercase;color:var(--teal);margin-bottom:9px}.intro>p{max-width:390px;color:var(--muted);font-size:16px;line-height:1.5}.choicebar{display:flex;align-items:center;justify-content:space-between;gap:16px;margin-bottom:16px}.relations{display:flex;gap:8px}.relations button{min-width:120px}.relations .sign{font:25px var(--math);vertical-align:-1px;margin-left:9px}.subtle{font-size:13px;color:var(--muted)}.stage{background:var(--panel);border:1px solid var(--line);border-radius:18px;box-shadow:var(--shadow);overflow:hidden;display:grid;grid-template-columns:minmax(265px,.9fr) minmax(0,1.8fr)}.algebra{padding:26px 30px;border-right:1px solid var(--line);display:flex;flex-direction:column;min-height:307px}.math{font-family:var(--math);font-size:37px;line-height:1.22;font-variant-numeric:lining-nums;white-space:nowrap;letter-spacing:.01em}.math i{font-style:italic}.original{font-size:48px;margin:6px 0 12px}.frac{display:inline-flex;vertical-align:middle;flex-direction:column;align-items:stretch;text-align:center;margin:0 .09em;font-size:.86em;line-height:1.08}.frac>span:first-child{border-bottom:1.4px solid currentColor;padding:0 .09em .09em}.frac>span:last-child{padding:.09em .09em 0}.operation{font-size:31px;min-height:77px;display:flex;align-items:center}.positive{color:var(--teal)}.solved{font-size:43px;font-weight:400;min-height:57px;display:flex;align-items:center}.placeholder{font:16px/1.45 'Segoe UI',Arial,sans-serif;color:var(--muted);white-space:normal}.math-label{color:var(--muted);font-size:12px;letter-spacing:.08em;text-transform:uppercase}.graph-area{padding:22px 25px 14px;min-width:0;display:flex;flex-direction:column;justify-content:center}.graph-top{display:flex;align-items:center;justify-content:space-between;gap:12px;font-size:13px;color:var(--muted)}.graph{display:block;width:100%;height:auto;overflow:visible}.graph .axis{stroke:#839490;stroke-width:1.8;fill:none}.graph .tick{stroke:#839490;stroke-width:1.5}.graph .tick-label{font:25px 'Segoe UI',Arial,sans-serif;fill:#435c57}.graph .var{font:italic 27px Georgia,serif;fill:var(--ink)}.graph .solution-ray{stroke:var(--teal);stroke-width:7;stroke-linecap:round;fill:none}.graph .ray-head{fill:var(--teal)}.graph .endpoint{stroke:var(--teal);stroke-width:3;fill:var(--teal)}.graph .endpoint.open{fill:#fff}.graph .boundary-marker{fill:#fff;stroke:var(--muted);stroke-width:2;stroke-dasharray:4 4}.graph .boundary-guide{stroke:#b8c8c2;stroke-width:1.6;stroke-dasharray:4 7}.graph .candidate{stroke:var(--gold);fill:#fff;stroke-width:3}.graph .candidate.good{stroke:var(--teal);fill:var(--wash)}.graph .candidate.bad{stroke:var(--rust);fill:#fbeee7}.graph .candidate-label{font:22px 'Segoe UI',Arial,sans-serif;fill:#6a4b19}.graph-foot{font-size:15px;color:var(--muted);min-height:43px}.graph-foot strong{color:var(--teal);font-weight:650}.insight{display:flex;gap:15px;align-items:start;padding:19px 3px 0;max-width:1020px;font-size:18px;line-height:1.55}.insight .mark{font:30px var(--math);color:var(--teal);line-height:1.1}.insight strong{font-weight:650}.three-up{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:16px}.compare-card{border:1px solid var(--line);background:#fff;border-radius:16px;padding:23px 20px}.compare-card h2{font-size:19px;margin-bottom:17px}.compare-card .math{font-size:33px;text-align:center}.compare-card .solved{justify-content:center;font-size:32px;margin-top:12px}.compare-card p{font-size:16px;color:var(--muted);margin-top:6px}.probe{margin-top:18px;border-top:1px solid var(--line);padding-top:15px}.probe-head{display:flex;gap:12px;align-items:center;flex-wrap:wrap}.probe-panel{margin-top:13px;border-radius:13px;background:#eeeee4;padding:17px;display:grid;grid-template-columns:1fr 1.4fr;gap:20px}.probe input[type=range]{width:100%;accent-color:var(--teal);height:30px}.probe label{display:block;font-size:14px;font-weight:600}.chips{display:flex;gap:7px;flex-wrap:wrap;margin:8px 0}.chips button{min-width:64px;padding:7px 13px}.probe-result .math{font-size:28px;white-space:normal}.truth{display:inline-block;font-family:'Segoe UI',Arial,sans-serif;font-size:16px;font-weight:700;border-radius:6px;padding:4px 9px;margin-top:8px;background:var(--wash);color:var(--teal)}.truth.false{background:#fae9e2;color:#993b25}.check-layout{display:grid;grid-template-columns:1fr 1fr;gap:20px}.prompt-card{background:#fff;border:1px solid var(--line);border-radius:18px;padding:26px 30px}.prompt-card h2{font-size:21px;margin-bottom:14px}.prompt-card .math{font-size:43px;margin-block:13px}.candidates{display:flex;gap:20px;margin:19px 0}.candidate-value{font:32px var(--math);border:2px solid transparent;padding:3px 12px;border-radius:50%}.candidate-value.chosen{border-color:var(--teal);color:var(--teal)}.prompt-card p{max-width:600px}.check-work{min-height:255px;display:flex;flex-direction:column;justify-content:center;gap:12px}.check-work .math{font-size:35px;white-space:normal}.solution-box{background:#fff;border:1px solid var(--line);border-radius:18px;padding:20px;margin-top:18px}.practice-select{display:flex;align-items:center;gap:14px;flex-wrap:wrap;margin-bottom:16px}.practice-select label{font-size:14px;font-weight:600}.practice-select select{min-width:215px}.practice-select .hint{margin-left:auto}.hint{font-size:14px;color:var(--muted);line-height:1.5}.controls{display:grid;grid-template-columns:minmax(220px,1fr) auto auto;gap:18px;align-items:center;padding:18px 40px;border-top:1px solid var(--line)}.step-info>span{font-size:11px;text-transform:uppercase;letter-spacing:.11em;color:var(--teal);font-weight:700}.step-info p{font-size:15px;margin-top:3px;max-width:510px}.transport,.motion-controls{display:flex;gap:7px;align-items:center}.transport #next{min-width:168px}.motion-controls{font-size:12px;flex-wrap:wrap}.motion-controls button,.motion-controls select{font-size:13px;padding-inline:11px}.motion-controls label{position:absolute;width:1px;height:1px;overflow:hidden;clip:rect(0,0,0,0)}.folio{display:flex;justify-content:space-between;gap:16px;padding:8px 40px 20px;font-size:11px;color:var(--muted)}dialog{border:1px solid var(--line);border-radius:20px;background:var(--paper);color:var(--ink);max-width:680px;width:calc(100% - 36px);max-height:90dvh;padding:30px;box-shadow:0 30px 90px #12342c35}dialog::backdrop{background:#1c3631b8;backdrop-filter:blur(5px)}dialog p,dialog ol{margin:18px 0}dialog li{margin-block:12px}.dialog-header{display:flex;align-items:start;justify-content:space-between;gap:16px}.dialog-header>button{font-size:25px;line-height:1;padding:6px 14px}.dialog-header .eyebrow{margin-top:10px}#paper-dialog{text-align:center;max-width:780px;padding:36px}#paper-dialog .math{margin-block:20px}#paper-dialog h2{font-size:40px}#paper-dialog #paper-prompt{font-size:21px;margin-top:25px}.paper-icon{font-size:64px;color:var(--teal);line-height:1.3}.skip{position:fixed;left:16px;top:-70px;z-index:10;padding:10px;background:white}.skip:focus{top:12px}.full-width{grid-column:1/-1}.screen-reader{position:absolute;clip:rect(0,0,0,0);width:1px;height:1px;overflow:hidden}.hidden{display:none!important}body[data-motion=still] *{scroll-behavior:auto;transition:none}body:fullscreen{overflow:auto}button.linklike{border:0;background:none;text-decoration:underline;text-underline-offset:4px;padding:6px;font-size:14px}
@media(min-width:1100px) and (max-height:800px){.masthead{padding-top:11px;padding-bottom:3px}.lesson-nav{padding-bottom:10px}main{padding-top:17px}.intro{margin-bottom:16px}h1{font-size:33px}.algebra{padding:18px 24px;min-height:262px}.original{font-size:42px;margin-bottom:6px}.operation{min-height:65px;font-size:28px}.solved{font-size:37px;min-height:48px}.graph-area{padding:14px 22px 10px}.graph{max-height:207px}.insight{padding-top:13px;font-size:17px}.controls{padding-top:12px;padding-bottom:12px}.probe{margin-top:12px;padding-top:8px}}
@media(max-width:1099px){.masthead,.lesson-nav,.controls,.folio{padding-inline:24px}main{padding-inline:24px}.intro{align-items:start}.intro>p{max-width:290px;font-size:15px}.controls{grid-template-columns:1fr auto}.motion-controls{grid-column:1/-1;justify-content:flex-end}.stage{grid-template-columns:minmax(225px,.9fr) minmax(0,1.6fr)}.algebra{padding:22px;min-height:285px}.original{font-size:40px}.operation{font-size:27px}.solved{font-size:36px}.relations button{min-width:0}.three-up{gap:10px}.compare-card{padding:20px 12px}.compare-card h2{font-size:17px}.compare-card .math{font-size:29px}.compare-card .solved{font-size:29px}.compare-card .graph{min-height:145px}.check-work .math{font-size:29px}.prompt-card{padding:23px}.utilities{font-size:13px}}
@media(max-width:760px){.masthead{align-items:start;padding:17px 17px 7px}.brand strong{font-size:17px}.brand small{font-size:10px}.brand svg{width:33px}.utilities{gap:0;flex-wrap:wrap;justify-content:flex-end;max-width:175px}.utilities button{font-size:12px;min-height:40px;padding:6px 9px}.lesson-nav{padding:9px 15px 13px;gap:3px;overflow-x:auto}.lesson-nav button{font-size:13px;padding:9px 11px;white-space:nowrap}.lesson-nav span{display:none}main{padding:21px 18px}.intro{display:block;margin-bottom:19px}.intro>p{max-width:none;margin-top:12px}.choicebar{align-items:start;flex-wrap:wrap}.relations{gap:5px}.relations button{font-size:13px;padding:8px 11px}.relations .sign{font-size:23px;margin-left:5px}.stage{grid-template-columns:1fr}.algebra{border-right:0;border-bottom:1px solid var(--line);min-height:0;padding:22px}.original{font-size:42px}.operation{font-size:29px;min-height:65px}.solved{min-height:48px}.graph-area{padding:18px 16px 8px}.graph{max-height:265px}.graph-foot{font-size:14px;min-height:38px}.graph-top{font-size:12px}.insight{font-size:17px}.three-up{grid-template-columns:1fr}.compare-card{padding:24px}.compare-card .math{font-size:34px}.compare-card .graph{max-height:210px}.compare-card .solved{font-size:32px}.check-layout{grid-template-columns:1fr}.check-work{min-height:190px}.controls{padding:16px 18px;grid-template-columns:1fr;gap:14px}.transport{justify-content:space-between}.transport button{flex:1}.motion-controls{justify-content:space-between}.folio{padding:7px 18px 17px;flex-wrap:wrap;gap:6px}.probe-panel{grid-template-columns:1fr}.practice-select .hint{margin-left:0}.intro h1{font-size:32px}#paper-dialog{padding:22px}#paper-dialog h2{font-size:31px}#paper-dialog #paper-prompt{font-size:18px}dialog{padding:23px}.choicebar>button{font-size:13px}.step-info p{font-size:15px}}
@media(prefers-reduced-motion:reduce){*{scroll-behavior:auto!important;transition:none!important}}
@media print{body{background:#fff}.masthead,.lesson-nav,.controls,.folio,.utilities,.probe{display:none}main{padding:0}.stage{box-shadow:none}.intro>p{max-width:none}dialog{display:none}}

/* Give the three comparison graphs readable labels at card size. */
.compare-card .graph{min-height:0;max-height:none;margin:28px 0 15px}.compact-graph .tick-label{font-size:45px}.compact-graph .var{font-size:40px}.compact-graph .solution-ray{stroke-width:9px}.compact-graph .endpoint{stroke-width:4px}.compare-card .solved{min-height:40px}
@media(max-width:760px){.compare-card .graph{margin:25px 0 16px}}

.motion-controls label{position:static;width:auto;height:auto;overflow:visible;clip:auto;font-size:12px;color:var(--muted)}
@media(max-width:760px){.relations{display:grid;grid-template-columns:repeat(3,1fr);width:100%}.relations button{line-height:1.25;min-height:70px}.relations .sign{display:block;margin:5px 0 0}.motion-controls{gap:6px}.motion-controls label{margin-left:auto}.motion-controls select{max-width:130px}}

.check-substitution{font-size:30px;margin-top:12px}.check-final .truth{align-self:flex-start;margin-top:18px}.check-final .graph-area{justify-content:center}
@media(min-width:1100px) and (max-height:800px){.compare-card{padding:17px 18px}.compare-card h2{margin-bottom:11px}.compare-card .solved{margin-top:8px;min-height:38px}.compare-card .graph{margin:14px 0 8px}.compare-card p{font-size:15px}.check-substitution{font-size:28px;margin-top:9px}.check-final .truth{margin-top:12px}.practice-select{margin-bottom:10px}.practice-select select{min-height:40px;padding-block:5px}}
@media(min-width:901px) and (max-width:1099px){.controls{grid-template-columns:minmax(170px,1fr) auto auto;gap:10px}.motion-controls{grid-column:auto;flex-wrap:nowrap;gap:5px}.step-info p{font-size:14px}.transport button{padding-inline:12px}.motion-controls button,.motion-controls select{padding-inline:8px}.motion-controls label{display:none}}
@media(max-width:1099px){.graph:not(.compact-graph) .tick-label{font-size:34px}.graph:not(.compact-graph) .var{font-size:33px}}
@media(max-width:760px){.masthead{flex-wrap:wrap;gap:4px}.brand{width:100%}.brand strong{white-space:nowrap}.brand small{display:block}.utilities{width:100%;max-width:none;justify-content:flex-start}.graph:not(.compact-graph) .tick-label{font-size:48px}.graph:not(.compact-graph) .var{font-size:43px}.graph:not(.compact-graph) .endpoint{r:16px;stroke-width:4px}.graph:not(.compact-graph) .candidate-label{font-size:36px}.graph:not(.compact-graph) .solution-ray{stroke-width:9px}.motion-controls{justify-content:flex-start;flex-wrap:nowrap}.motion-controls label{display:none}.motion-controls button,.motion-controls select{font-size:12px;padding-inline:8px}.motion-controls select{margin-left:auto}.check-final .graph-foot{margin-top:15px}}

.inspect-instruction{display:block;margin-top:8px;font-size:14px;line-height:1.4;color:var(--teal)}
.probe-result:focus-visible{outline:3px solid #bd742c;outline-offset:6px;border-radius:4px}
@media(max-width:760px){.lesson-nav{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));overflow:visible;gap:4px}.lesson-nav button{white-space:normal;text-align:center}}