.geometry-lesson {




& {color-scheme:only light;--paper:#fbfaf7;--surface:#fff;--surface-soft:#f3f0e9;--surface-green:#eef4f0;--ink:#282a27;--muted:#6f726c;--line:#dedfd9;--accent:#355647;--accent-dark:#355647;--soft:#eef4f0;--orange:#dc743a;--success:#2f623e;--error:#86502a;--font-display:"Lora",Georgia,serif;--font-body:"Golos Text",-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;font-size:16px}*{box-sizing:border-box}& {-webkit-text-size-adjust:100%;background:var(--paper)}& {margin:0;background:var(--paper);color:var(--ink);font-family:var(--font-body);font-size:17px;line-height:1.55;-webkit-font-smoothing:antialiased}button,input,select{font:inherit}button{cursor:pointer}button:disabled{cursor:default;opacity:.5}button,input,select{-webkit-tap-highlight-color:transparent}:focus-visible{outline:3px solid #d78b4d;outline-offset:4px}[hidden]{display:none!important}a{color:var(--accent-dark);text-decoration:none}a:hover{text-decoration:underline}h1,h2{font-family:var(--font-display);font-weight:600;letter-spacing:-.02em}h1,h2,h3,p{margin-top:0}p:last-child{margin-bottom:0}h1{margin:0 0 9px;font-size:clamp(32px,5vw,46px);line-height:1.15}h1 span{color:var(--accent)}h2{font-size:23px;line-height:1.3;margin:0 0 8px}h3{font-size:18px;margin:0 0 10px;line-height:1.4;font-weight:600;letter-spacing:-.02em}.site-header{border-bottom:1px solid var(--line);background:rgba(251,250,247,.96)}.header-inner{width:min(calc(100% - 40px),1320px);margin:auto;min-height:72px;display:flex;justify-content:space-between;align-items:center;gap:24px}.brand{font-family:var(--font-display);font-size:1.2rem;font-weight:600;line-height:1.3}.header-context{color:var(--muted);font-size:.875rem}.shell{max-width:1000px;padding:24px 30px 32px;margin:auto;min-width:0}.lesson-heading{padding:20px 0 24px}.lesson-heading>p{color:var(--muted);margin:0}.tabs{display:flex;gap:6px;border-bottom:1px solid var(--line);margin:0 0 25px}.tab{position:relative;border:0;padding:12px 19px 15px;background:transparent;color:var(--muted);font-weight:600;white-space:nowrap;line-height:1.5}.tab-num{font-size:12px;opacity:.65;margin-right:7px}.tab[aria-selected=true]{color:var(--accent)}.tab[aria-selected=true]:after{position:absolute;content:"";height:3px;border-radius:3px 3px 0 0;background:var(--accent);bottom:-1px;left:16px;right:16px}.card{background:var(--surface);border:1px solid var(--line);border-radius:16px;padding:27px 30px;min-width:0}.card-head{display:flex;align-items:flex-start;justify-content:space-between;gap:20px}.lead,.muted{color:var(--muted)}.lead{margin:0;max-width:610px;font-size:17px;line-height:1.55}.demo-card .readout{flex-shrink:0;background:var(--surface-green);border-radius:12px;padding:9px 15px;max-width:310px}.demo-card .readout .big{font-size:20px;font-weight:600;line-height:1.45;white-space:normal;font-variant-numeric:tabular-nums}.model-note{color:var(--muted);font-size:13px;margin:10px 0 0;max-width:620px}.board{border-radius:15px;padding:12px 14px;background-color:#fafbf7;background-image:linear-gradient(#e9ede480 1px,transparent 1px),linear-gradient(90deg,#e9ede480 1px,transparent 1px);background-size:24px 24px;min-width:0}.lab-grid{display:block}.lab-grid>.board{margin-top:24px}.diagram{display:block;width:100%;height:360px;overflow:visible;touch-action:pan-y}.diagram text{font-family:var(--font-body);fill:var(--ink)}.diagram .angle-label{font-size:22px;font-weight:600}.diagram .small-label{font-size:16px;font-weight:600}.diagram .muted-label{font-size:14px;fill:var(--muted)}.latex-label{pointer-events:none}.handle{cursor:grab;touch-action:none}.handle:active{cursor:grabbing}.controls{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:18px 24px;margin-top:21px}.controls>.slider-area{grid-column:1/-1}.controls>div:has(.chips){grid-column:1/-1}.controls>div:has(.step-nav){grid-column:1/-1}.controls>.model-note{grid-column:1/-1}.slider-area{max-width:620px;width:100%;margin:0 auto}.control label{display:block;font-weight:600;font-size:16px;margin-bottom:8px}.slider-row{display:flex;align-items:center;gap:15px}.slider-row input[type=range]{accent-color:var(--accent);flex:1;min-width:0;cursor:ew-resize}.step-button{width:42px;height:42px;border:1px solid var(--line);border-radius:12px;background:var(--surface);color:var(--ink);font-size:24px;line-height:1}.step-button:hover{background:var(--paper)}.range-ends{display:flex;justify-content:space-between;font-size:12px;color:var(--muted);margin:3px 57px 0}.numeric-control{display:flex;justify-content:center;align-items:center;gap:9px;font-size:14px;color:var(--muted);margin-top:10px}.numeric-control input{width:84px;padding:6px 9px;min-height:36px}.numeric-control label{font-size:14px;font-weight:400;margin:0}select,input[type=number],input[type=text]{border:1px solid #c5d0c4;border-radius:11px;padding:9px 11px;background:white;color:var(--ink);max-width:100%;min-height:44px;font-variant-numeric:tabular-nums}.control select{width:100%}.control:not(.slider-area)>label{font-size:14px}.chips{display:flex;gap:8px;flex-wrap:wrap;justify-content:center}.chip{padding:6px 13px;min-height:36px;border:1px solid var(--line);border-radius:10px;background:white;color:var(--muted);font-size:14px}.chip:hover{background:var(--paper);color:var(--accent)}.chip.active{background:var(--surface-green);border-color:#b9d1b7;color:var(--accent)}.tag{font-size:13px;border:1px solid var(--line);border-radius:8px;background:var(--surface);padding:4px 8px;color:var(--muted)}.btn-row,.actions{display:flex;gap:12px;align-items:center;flex-wrap:wrap;margin-top:18px}.btn,.primary,.secondary,.quiet{border-radius:12px;min-height:44px;padding:10px 18px;font-weight:600;font-size:16px;line-height:1.45}.btn{border:1px solid var(--line);background:white;color:var(--ink);text-decoration:none;display:inline-flex;align-items:center;justify-content:center}.btn:hover{background:var(--paper);text-decoration:none}.btn.primary,.primary{border:1px solid var(--accent);background:var(--accent);color:white}.btn.primary:hover,.primary:hover{background:#294538}.btn.ghost,.quiet{border:0;background:transparent;color:var(--muted)}.btn.ghost:hover,.quiet:hover{color:var(--accent);background:#f0f5ed}.btn.secondary,.secondary{border:1px solid var(--line);background:white;color:var(--ink)}.rules{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:22px;margin:25px 0}.rule{border-left:2px solid #cbdcc8;padding-left:15px;min-width:0}.rule b{display:block;font-size:15px;margin-bottom:5px}.rule p{margin:0;color:var(--muted);font-size:14px}.rule .formula{display:block;color:var(--accent);font-size:16px;font-weight:600;margin:6px 0 0}.formula{font-size:20px;color:var(--accent);font-weight:600;line-height:1.4;margin-top:12px}.grid{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:18px}.grid.three{grid-template-columns:repeat(3,minmax(0,1fr))}.grid .card p{color:var(--muted);font-size:16px}.card-icon{display:inline-flex;align-items:center;justify-content:center;min-width:46px;padding:8px 13px;border-radius:10px;background:var(--surface-green);color:var(--accent);font-size:16px;font-weight:600;margin-bottom:14px}.callout{background:var(--surface-green);border-left:2px solid #cbdcc8;padding:18px 20px;border-radius:0 12px 12px 0;margin:20px 0}.callout h3{font-size:17px}.callout p{font-size:16px}.callout li{font-size:16px}.example{display:grid;grid-template-columns:1.1fr 1fr;gap:24px;align-items:center;margin:20px 0}.example li,.steps li{margin-bottom:9px}.eyebrow{font-size:12px;font-weight:600;letter-spacing:.08em;text-transform:uppercase;color:var(--accent);margin-bottom:7px}.book-picture{margin:12px 0 18px;min-width:0}.book-picture .latex-figure{display:block;width:100%;height:auto;max-height:330px}.book-picture figcaption{font-size:13px;text-align:center;color:var(--muted);margin-top:8px}.image-pair{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:8px}.image-pair .book-picture{margin:12px 0}.theory-diagram .book-picture{margin:0}.book-angle-grid .latex-figure{max-height:205px}.view-head{display:flex;align-items:baseline;justify-content:space-between;gap:16px;margin-bottom:18px}.view-head h2{margin:0}.note{font-size:14px;color:var(--muted)}details{padding:14px 0;border-top:1px solid var(--line);margin-top:12px}summary{cursor:pointer;font-size:16px;font-weight:600}details p{margin:12px 0 0;font-size:16px}.practice-switch{display:flex;gap:8px;flex-wrap:wrap;margin:0 0 20px}.practice-switch button{background:white;border:1px solid var(--line);border-radius:10px;padding:8px 14px;min-height:42px;font-size:15px;color:var(--muted)}.practice-switch button.active{background:var(--surface-green);border-color:#b9d1b7;color:var(--accent);font-weight:600}.stats{display:flex;gap:14px;flex-wrap:wrap;font-size:14px;color:var(--muted)}.stats strong{color:var(--ink)}.practice-layout{display:flex;flex-direction:column;background:var(--surface);border:1px solid var(--line);border-radius:16px;padding:27px 30px}.practice-layout>.board{order:2;margin:20px 0}.practice-layout>.card{display:contents}.practice-layout>.card>.eyebrow{order:0}.practice-layout>.card>.question-text{order:1}.practice-layout>.card>form{order:3}.practice-layout>.card>.btn-row{order:4}.practice-layout>.card>#practice-help{order:5}.practice-layout>.card>#practice-feedback{order:6}.practice-layout>.card>hr{order:7}.practice-layout>.card>#practice-next{order:8;align-self:flex-end}.question-text{font-size:18px;line-height:1.55;font-weight:500;margin:0}.answer-line{display:flex;gap:12px;align-items:center;flex-wrap:wrap}.answer-line label{font-size:15px;font-weight:600}.answer-line input{width:110px}.answer-options{display:flex;gap:9px;flex-wrap:wrap}.option{display:flex;align-items:center;gap:9px;padding:10px 14px;border:1px solid #c5d0c4;border-radius:12px;cursor:pointer;font-size:15px;min-height:44px}.option:hover{background:var(--paper)}.option:has(input:checked){background:var(--surface-green);border-color:var(--accent);color:var(--accent)}.option input{accent-color:var(--accent);width:16px;height:16px;flex-shrink:0;margin:0}.feedback{margin-top:17px;padding:13px 16px;border-radius:12px;font-size:15px;background:var(--surface-green);min-width:0}.feedback:empty{display:none}.feedback.good{background:#edf6e9;color:#2f623e}.feedback.bad{background:#fff1e3;color:#86502a}.feedback p{margin:6px 0 0}.hint{margin-top:14px;padding-left:13px;border-left:2px solid var(--orange);color:var(--muted);font-size:14px}.divider{border:0;border-top:1px solid var(--line);margin:20px 0}.quiz-top{display:flex;align-items:center;justify-content:space-between;gap:20px;margin-bottom:18px}.quiz-links{display:flex;gap:7px;flex-wrap:wrap}.task-link{width:36px;height:38px;border:1px solid var(--line);border-radius:10px;background:white;color:var(--muted);font-size:14px}.task-link[aria-current=step]{border-color:var(--accent);background:var(--accent);color:white}.task-link.solved:not([aria-current=step]){color:var(--accent);background:var(--surface-green);border-color:#cbdcc8}.quiz-progress{display:flex;align-items:center;gap:12px;color:var(--muted);font-size:14px;white-space:nowrap}.progress{height:6px;width:94px;border-radius:9px;background:#e1e8dc;overflow:hidden}.progress>div{height:100%;background:var(--accent);transition:width .2s}.quiz-wrap .board{margin:20px 0}.quiz-wrap .question-text{margin-bottom:18px}.quiz-footer{display:flex;justify-content:space-between;align-items:center;gap:12px;margin-top:25px;padding-top:19px;border-top:1px solid var(--line)}.quiz-score{font-size:48px;font-weight:600;line-height:1;color:var(--accent);margin:22px 0}.review-item{padding:18px 0;border-top:1px solid var(--line)}.review-item p{margin:6px 0}.review-item .result-label{font-size:14px;color:var(--error);font-weight:600}.review-item.correct .result-label{color:var(--success)}.readout{background:var(--surface-green);border-radius:12px;padding:15px}.readout .big{font-size:22px;font-weight:600}.readout p{color:var(--muted);font-size:14px;margin:8px 0 0}.step-nav{display:flex;gap:7px;margin:0 0 15px}.step-nav button{width:40px;height:38px}.all-angle-values{border-top:1px solid var(--line);padding-top:18px;margin-top:20px}.all-angle-values h3{font-size:17px}.lesson-footer{margin-top:32px;border-top:1px solid var(--line);padding-top:18px;color:var(--muted);font-size:13px;display:flex;justify-content:space-between;gap:12px}.site-footer{border-top:1px solid var(--line);font-size:14px;color:var(--muted);padding:24px 20px;margin-top:36px}.site-footer>div{width:min(100%,1320px);margin:auto;display:flex;justify-content:space-between;gap:12px}.skip{position:absolute;top:-100px;left:20px;z-index:10;background:white;padding:12px}.skip:focus{top:12px}.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}.error-message{padding:18px;border-radius:12px;background:#fff1e3;color:var(--error)}
@media(max-width:700px){.shell{padding:22px 17px}.header-inner{min-height:62px;gap:12px}.brand{font-size:18px}.header-context{font-size:12px}.lesson-heading{padding:18px 0 20px}.card,.practice-layout{padding:21px 18px;border-radius:16px}.card-head{flex-wrap:wrap;gap:12px}.demo-card .readout .big{font-size:18px}.demo-card .readout{padding:7px 12px}h2{font-size:21px}.board{padding:9px 5px}.lab-grid>.board{margin-top:19px}.diagram{height:285px}.rules{grid-template-columns:1fr;gap:17px}.controls{grid-template-columns:1fr;gap:18px}.controls>.slider-area,.controls>div:has(.chips),.controls>div:has(.step-nav),.controls>.model-note{grid-column:auto}.grid,.grid.three,.example{grid-template-columns:1fr}.quiz-top{align-items:flex-start;flex-direction:column;gap:14px}.quiz-progress{width:100%;justify-content:space-between}.progress{flex:1;max-width:180px}.task-link{width:40px;height:40px}.view-head{flex-wrap:wrap;gap:12px}.image-pair .latex-figure{max-height:260px}.question-text{font-size:17px}.stats{gap:10px}.site-footer>div{flex-wrap:wrap}}
@media(max-width:430px){.tabs{gap:0}.tab{padding:12px 10px 15px;font-size:14px}.tab[aria-selected=true]:after{left:8px;right:8px}.tab-num{font-size:11px;margin-right:4px}.header-context{display:none}.slider-row{gap:10px}.range-ends{margin-left:52px;margin-right:52px}.diagram{height:245px}.option{padding:10px 12px}.quiz-footer .btn{padding-left:13px;padding-right:13px}.btn{font-size:15px}.lesson-footer{flex-wrap:wrap}}
@media(pointer:coarse){.step-button,.task-link{min-width:44px;min-height:44px}.chip{min-height:44px}}
@media(prefers-reduced-motion:reduce){*{transition:none!important;scroll-behavior:auto!important}}
@media print{& {background:white}.site-header,.site-footer,.tabs,.btn-row,.practice-switch,.skip,.controls{display:none}.shell{max-width:none;padding:0}.view[hidden]{display:block!important}.view{break-before:page}.card,.board,.book-picture{break-inside:avoid}}
.diagram .handle:not([fill="transparent"]){fill:var(--orange)}

.lesson-links {display:flex;justify-content:space-between;gap:16px;flex-wrap:wrap;margin-top:28px;border-top:1px solid var(--line);padding-top:18px;} .units-extra {margin-top:24px;} .converter-grid {display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:18px;margin:20px 0;} .converter-fields {display:flex;gap:12px;margin:18px 0;flex-wrap:wrap;} .converter-field {flex:1;min-width:70px;} .field-label {display:block;font-size:14px;margin-bottom:6px;} .field-with-unit {display:flex;align-items:center;gap:6px;} .number-input {width:100%;border:1px solid var(--line);border-radius:10px;padding:8px;min-height:44px;} .result-grid {display:grid;gap:12px;} .result-label {display:block;color:var(--muted);font-size:14px;} output {font-size:22px;font-weight:600;color:var(--accent);} .calculation {font-size:14px;margin-top:12px;} .converter-error {color:var(--error);} @media(max-width:700px){.converter-grid{grid-template-columns:1fr;}} }