@import url('/ax-tokens.css');
@font-face{font-family:'Source Sans 3';src:url('/fonts/SourceSans3-Regular.woff2') format('woff2');font-weight:400;font-display:swap}
@font-face{font-family:'Source Sans 3';src:url('/fonts/SourceSans3-Semibold.woff2') format('woff2');font-weight:600;font-display:swap}
@font-face{font-family:'Source Serif 4';src:url('/fonts/SourceSerif4-Regular.woff2') format('woff2');font-weight:400;font-display:swap}
:root{--blue:var(--ax-blue);--teal:var(--ax-welcome-teal);--ink:var(--ax-text);--muted:var(--ax-muted);--paper:var(--ax-reading);--wash:var(--ax-page);--line:var(--ax-rule);font-family:'Source Sans 3','Microsoft YaHei',sans-serif;font-size:18px;line-height:1.55;color:var(--ink);background:var(--ax-page);color-scheme:light}
*{box-sizing:border-box}body{margin:0}a{color:inherit;text-decoration:none}button,input{font:inherit}button{cursor:pointer}button,a,input,summary{touch-action:manipulation}button:focus-visible,a:focus-visible,input:focus-visible,summary:focus-visible{outline:3px solid #007c69;outline-offset:5px}button,a{transition:background-color .18s,color .18s,border-color .18s}a:hover{color:var(--ax-link)}[hidden]{display:none!important}svg{display:block}.page-width{width:min(1280px,calc(100% - 112px));margin-inline:auto}.skip-link{position:absolute;left:20px;top:-80px;z-index:20;background:var(--ax-reading);padding:12px}.skip-link:focus{top:10px}
.site-header{min-height:118px;display:flex;align-items:center;justify-content:space-between;gap:36px;padding:22px max(40px,calc((100vw - 1360px)/2));position:relative;background:var(--ax-reading);color:var(--ax-graphite);border-bottom:1px solid var(--ax-rule)}.brand{display:flex;flex-direction:column;gap:5px;flex-shrink:1;min-width:0}.wordmark{font:400 39px/1.1 'Source Serif 4',Georgia,serif;letter-spacing:.055em;color:var(--ax-blue)}.brand-dot{color:#008c75}.brand-sub{font-size:12px;line-height:1.4;max-width:345px}.desktop-nav{display:flex;gap:30px;font-size:16px;font-weight:600;white-space:nowrap}.header-actions{display:flex;gap:25px;align-items:center;flex-shrink:0}.languages{display:flex;align-items:center;gap:7px;font-size:13px}.languages button{background:none;border:0;min-height:40px;padding:5px 2px;color:var(--muted);white-space:nowrap}.languages button[aria-pressed=true]{color:var(--ax-blue);text-decoration:underline;text-underline-offset:5px}.header-link{font-size:15px;font-weight:600;border-bottom:2px solid var(--ax-blue);padding-block:7px;color:var(--ax-link)}.menu-toggle{display:none}.mobile-nav{position:absolute;left:0;right:0;top:100%;z-index:10;background:var(--ax-reading);border-top:1px solid var(--line);border-bottom:1px solid var(--line);padding:16px 24px}.mobile-nav a{display:block;padding:13px}
.math-hero{margin:0 32px;background:var(--blue);position:relative;min-height:660px;isolation:isolate;overflow:hidden}.hero-geometry{position:absolute;inset:0;width:100%;height:100%;z-index:-1}.geometry-axis{stroke:#8ba9eb;stroke-opacity:.25;fill:none;stroke-width:1}.geometry-equation{fill:var(--teal);font:italic 38px 'Source Serif 4',serif}.small-equation{fill:#c0d6ff;font:16px 'Source Sans 3',sans-serif}.hero-copy{padding-top:112px;padding-bottom:108px;width:710px;max-width:58%}.hero-title-block{background:var(--ax-reading);padding:24px 32px 25px}.hero-title-block h1{font-size:clamp(58px,5.5vw,80px);line-height:1.04;letter-spacing:-.025em;font-weight:650;margin:0;color:var(--ax-graphite)}.hero-title-block h1 span{display:block}.hero-action-block{background:var(--teal);margin-top:22px;padding:25px 32px 28px;max-width:580px;color:#082d32}.hero-action-block p{font-size:19px;line-height:1.5;max-width:44ch;margin:0 0 20px}.button{display:inline-flex;align-items:center;justify-content:center;gap:28px;min-height:50px;padding:13px 22px;font-size:16px;font-weight:600;border:1px solid transparent}.button svg,.entry-links svg{width:21px;height:21px;fill:none;stroke:currentColor;stroke-width:1.7;flex:none}.button-ink{background:var(--ax-graphite);color:var(--ax-reading)}.button-ink:hover{background:var(--ax-blue-hover);color:var(--ax-reading)}.button-outline{border-color:#89a9e6;color:var(--ax-reading);background:transparent}.button-outline:hover{background:var(--teal);border-color:var(--teal);color:var(--ink)}.hero-motion-bar{position:absolute;bottom:22px;left:32px;right:28px;display:flex;justify-content:space-between;align-items:center;gap:20px;color:var(--ax-reading);font-size:13px}.hero-motion-bar>span{color:#c9daff}.hero-motion-bar button{display:flex;align-items:center;gap:10px;border:1px solid var(--ax-selected-border);background:var(--ax-blue);min-height:42px;padding:8px 14px;color:var(--ax-reading);font-size:13px;white-space:nowrap}.hero-motion-bar button:hover{background:var(--ax-blue-hover)}.hero-motion-bar button:focus-visible{outline-color:var(--teal)}.motion-symbol{font-weight:700;font-size:16px;min-width:12px}
.entry-links{display:grid;grid-template-columns:1fr 1fr;gap:24px;margin-top:28px}.entry-links a{display:flex;justify-content:space-between;align-items:center;gap:15px;padding:24px 30px;background:var(--teal);color:#092f32;font-size:24px;font-weight:600;min-height:90px}.entry-links a:nth-child(2){background:var(--ax-selected);color:var(--ax-blue)}.entry-links a:hover{background:var(--blue);color:var(--ax-reading)}.intro{display:grid;grid-template-columns:.85fr 1.15fr;gap:95px;padding-block:85px 80px}.intro h2,.section-heading h2,.approach-heading h2,.experience h2,.questions h2,.closing h2{font-size:clamp(38px,3.8vw,54px);line-height:1.09;letter-spacing:-.025em;font-weight:600;margin:0}.intro p{font-size:18px;color:var(--muted);margin:0 0 17px;max-width:59ch}.intro .intro-lead{font-size:21px;line-height:1.5;color:var(--ink)}.intro .field-line{font-size:14px;line-height:1.9;border-top:1px solid var(--line);padding-top:18px;margin-top:22px;color:var(--blue)}
.courses-section{background:var(--wash);padding-block:76px}.section-heading{display:flex;justify-content:space-between;align-items:center;gap:60px}.section-heading p{font-size:18px;line-height:1.6;max-width:43ch;color:var(--muted);margin:0}.course-toolbar{display:flex;justify-content:space-between;gap:20px;align-items:center;margin-top:38px;margin-bottom:26px}.course-filters{display:flex;gap:8px;flex-wrap:wrap}.course-filters button{padding:10px 18px;border:1px solid var(--ax-button-border);background:transparent;color:var(--ink);font-size:15px;min-height:44px}.course-filters button[aria-pressed=true]{background:var(--blue);border-color:var(--blue);color:white}.course-filters button:hover{border-color:var(--blue)}#course-count{font-size:14px;color:var(--muted);white-space:nowrap}.course-list{display:grid;grid-template-columns:1fr 1fr;gap:24px}.module-card{display:flex;flex-direction:column;background:var(--ax-reading);border:1px solid var(--ax-rule);padding:29px 32px 25px;min-width:0;position:relative;min-height:314px;box-shadow:none}.module-card:hover{border-color:var(--ax-blue);background:var(--ax-control-hover)}.module-subject{font-size:14px;color:var(--ax-blue);font-weight:600;margin-bottom:13px}.module-card h3{font-size:32px;line-height:1.15;font-weight:600;letter-spacing:-.018em;margin:0 0 15px}.module-question{font-size:18px;line-height:1.5;margin:0 0 20px;color:var(--muted)}.module-foundations{font-size:15px;line-height:1.55;color:var(--muted);margin:0 0 25px}.module-foundations strong{color:var(--ink);font-weight:600}.module-entry{margin-top:auto;display:flex;align-items:center;justify-content:space-between;border-top:1px solid var(--ax-rule);padding-top:17px;font-size:15px;font-weight:600;color:var(--ax-link)}.module-entry svg{width:19px;height:19px;stroke:currentColor;stroke-width:1.6;fill:none}.course-footer{display:flex;align-items:center;justify-content:space-between;gap:20px;margin-top:24px}.course-footer p{font-size:14px;color:var(--muted);margin:0}.text-link{display:inline-flex;align-items:center;gap:20px;border-bottom:1px solid var(--ax-link);padding-bottom:3px;color:var(--ax-link);font-size:16px;font-weight:600;white-space:nowrap}.course-error{grid-column:1/-1;padding:20px 0}.course-error button{background:var(--blue);color:white;border:0;padding:12px 20px}
.approach-section{background:var(--blue);color:white;padding-block:78px}.approach-layout{display:grid;grid-template-columns:1fr 1fr;gap:100px;align-items:start}.approach-heading h2{font-size:52px}.approach-heading p{font-size:19px;line-height:1.7;color:#c9daff;max-width:37ch;margin:24px 0 30px}.method-list{list-style:none;padding:0;margin:0}.method-list li{display:flex;gap:25px;border-top:1px solid #4d6fba;padding:25px 0}.method-list li:last-child{border-bottom:1px solid #4d6fba}.method-list li>span{font-size:18px;color:var(--teal);padding-top:5px}.method-list h3{font-size:28px;line-height:1.2;font-weight:600;margin:0 0 11px}.method-list p{font-size:17px;color:#c9daff;line-height:1.65;margin:0;max-width:43ch}
.experience{display:grid;grid-template-columns:.87fr 1.13fr;gap:100px;align-items:center;padding-block:80px 44px}.experience-copy>p{font-size:20px;line-height:1.65;color:var(--muted);max-width:35ch;margin:25px 0}.experience-copy .experience-note{font-size:14px;color:var(--blue)}.experiment{margin:0;min-width:0}.experiment-heading{display:flex;justify-content:space-between;align-items:baseline;gap:20px}.experiment-heading h2{font:600 15px/1.4 'Source Sans 3',sans-serif;letter-spacing:0;max-width:30ch;margin:0}.function-label{font:italic 22px 'Source Serif 4',serif;white-space:nowrap}#graph{display:block;width:100%;overflow:visible;margin-top:10px;user-select:none}.grid path{stroke:var(--ax-rule);fill:none;stroke-width:.8}.axes path{fill:none;stroke:#8191a5;stroke-width:1}.axis-labels{font-size:12px;fill:var(--muted)}.curve{fill:none;stroke:var(--blue);stroke-width:2.4}.tangent{fill:none;stroke:#8795a6;stroke-width:1.3;stroke-dasharray:5 5}.secant{fill:none;stroke:#008d75;stroke-width:2.2}.projection{fill:none;stroke:#82b7b0;stroke-width:1;stroke-dasharray:3 5}.fixed-point{fill:var(--blue);stroke:var(--ax-reading);stroke-width:2}.moving-point{fill:#008d75;stroke:var(--ax-reading);stroke-width:2;pointer-events:none}.moving-halo{fill:#008d75;fill-opacity:.14;cursor:ew-resize;touch-action:none}.point-label{font:14px 'Source Serif 4',serif;fill:var(--ink)}.moving-label{fill:#007c69}.curve-label{font:italic 18px 'Source Serif 4',serif;fill:var(--blue)}.experiment-controls{margin-inline:18px}.slider-label{display:flex;justify-content:space-between;align-items:center;gap:12px;font-size:14px}.slider-label output{white-space:nowrap;font-variant-numeric:tabular-nums;color:var(--blue)}input[type=range]{width:100%;height:30px;margin:10px 0 0;accent-color:#008d75;cursor:ew-resize}.range-ends{display:flex;align-items:center;justify-content:space-between;font-size:12px;color:var(--muted)}.range-ends button{border:0;border-bottom:1px solid #9daabe;color:var(--blue);padding:6px 10px;background:none;font-size:13px;min-height:36px}.slope-row{display:flex;align-items:center;justify-content:space-between;border-top:1px solid var(--line);padding-top:10px;margin-top:13px;font-size:14px}.slope-row output{font-size:32px;line-height:1.3;color:var(--blue);font-variant-numeric:tabular-nums}#graph-instruction,.limit-note{font-size:13px;line-height:1.6;color:var(--muted);margin:8px 0}.limit-note{color:var(--ink)}.experiment figcaption{display:flex;justify-content:space-between;gap:12px;margin:18px 18px 0;padding-top:12px;border-top:1px solid var(--line);font-size:12px;color:var(--muted)}#slope-formula{font-family:'Source Serif 4',serif;white-space:nowrap}
.surface-extra{margin-bottom:58px}.surface-extra details{border-block:1px solid var(--line)}summary{cursor:pointer;list-style:none;font-size:18px;font-weight:600;padding:22px 38px 22px 0;position:relative}summary::-webkit-details-marker{display:none}summary:after{content:'+';position:absolute;right:3px;top:16px;font-size:28px;font-weight:400}details[open]>summary:after{content:'−'}.surface-content{display:grid;grid-template-columns:.8fr 1.2fr;gap:50px;padding-bottom:30px}.surface-content p{font-size:18px;color:var(--muted);margin:0;max-width:40ch}.surface-content video{display:block;width:100%;aspect-ratio:16/9;background:#101f33}.questions{display:grid;grid-template-columns:.78fr 1.22fr;gap:80px;padding-block:36px 78px}.questions h2{font-size:42px}.faq-list details{border-top:1px solid var(--line)}.faq-list details:last-child{border-bottom:1px solid var(--line)}.faq-list p{font-size:17px;line-height:1.7;color:var(--muted);margin:0 20px 24px 0}.closing{background:var(--teal);padding-block:57px}.closing-inner{display:flex;align-items:center;justify-content:space-between;gap:35px}.closing h2{font-size:52px}.site-footer{display:flex;align-items:center;justify-content:space-between;gap:25px;min-height:116px;font-size:13px;color:var(--muted)}.footer-brand{font:21px 'Source Serif 4',serif;color:var(--blue)}
html[lang=zh-CN]{font-family:'Microsoft YaHei','PingFang SC',sans-serif}html[lang=zh-CN] .hero-title-block h1{font-size:62px;line-height:1.2;font-weight:600}html[lang=zh-CN] h2{letter-spacing:0;line-height:1.25}html[lang=zh-CN] .module-card h3{font-size:29px}html[lang=zh-CN] .method-list h3{font-size:25px}
@media(max-width:1160px){.site-header{padding-inline:32px;gap:22px}.desktop-nav{gap:20px;font-size:15px}.header-actions{gap:18px}.brand-sub{max-width:275px;font-size:11px}.page-width{width:calc(100% - 80px)}.hero-copy{max-width:65%}.hero-title-block h1{font-size:64px}.hero-action-block{max-width:520px}.intro,.approach-layout,.experience{gap:55px}.entry-links a{font-size:21px}.approach-heading h2{font-size:44px}}
@media(max-width:850px){.site-header{min-height:100px;padding:19px 24px;gap:18px}.wordmark{font-size:31px}.brand-sub{max-width:255px;font-size:11px}.desktop-nav,.header-link{display:none}.menu-toggle{display:block;background:transparent;border:1px solid var(--line);padding:9px 12px;font-size:14px;min-height:40px}.header-actions{gap:13px}.page-width{width:calc(100% - 48px)}.math-hero{margin:0 16px;min-height:0}.hero-copy{max-width:none;width:100%;padding-top:50px;padding-bottom:295px}.hero-title-block{width:calc(100% - 26px);padding:22px 24px}.hero-title-block h1{font-size:clamp(38px,6.8vw,58px);line-height:1.05}.hero-action-block{width:calc(100% - 50px);max-width:520px;padding:21px 24px;margin-top:18px}.hero-action-block p{font-size:18px;line-height:1.5}.hero-geometry{inset:auto 0 40px;width:100%;height:345px}.hero-motion-bar{left:20px;right:18px;bottom:16px;font-size:11px;gap:12px}.hero-motion-bar>span{max-width:22ch}.hero-motion-bar button{font-size:12px;min-height:40px;padding:7px 10px}.entry-links{gap:14px;margin-top:18px}.entry-links a{font-size:18px;line-height:1.3;padding:19px 20px;min-height:92px}.entry-links svg{width:18px}.intro{grid-template-columns:1fr;gap:26px;padding-block:48px}.intro h2,.section-heading h2,.experience h2{font-size:40px}.intro .intro-lead{font-size:20px}.section-heading{display:block}.section-heading p{margin-top:24px}.courses-section{padding-block:48px}.course-toolbar{align-items:flex-start;margin-top:28px;gap:14px;flex-direction:column}.course-filters{gap:7px}.course-filters button{font-size:14px;padding:9px 13px}.course-list{grid-template-columns:1fr}.module-card{min-height:0;padding:24px}.module-card h3{font-size:31px}.module-question{font-size:18px}.module-foundations{font-size:15px}.course-footer{align-items:flex-start;flex-direction:column;gap:15px}.approach-section{padding-block:48px}.approach-layout{grid-template-columns:1fr;gap:38px}.approach-heading h2{font-size:43px}.method-list h3{font-size:26px}.experience{grid-template-columns:1fr;gap:34px;padding-block:50px 35px}.experience-copy>p{font-size:19px;margin:22px 0}.experiment-heading h2{font-size:14px}.experiment-controls{margin-inline:5px}.experiment figcaption{margin-inline:5px;font-size:11px}.slider-label{font-size:13px}.surface-extra{margin-bottom:30px}.surface-content{grid-template-columns:1fr;gap:24px}.surface-extra summary{font-size:16px}.questions{grid-template-columns:1fr;gap:27px;padding-block:25px 50px}.questions h2{font-size:36px}.faq-list summary{font-size:17px}.closing{padding-block:42px}.closing-inner{align-items:flex-start;flex-direction:column;gap:26px}.closing h2{font-size:43px}.site-footer{flex-wrap:wrap;padding-block:28px;gap:14px}.footer-brand{width:100%}html[lang=zh-CN] .hero-title-block h1{font-size:42px}html[lang=zh-CN] .intro h2,html[lang=zh-CN] .section-heading h2,html[lang=zh-CN] .experience h2,html[lang=zh-CN] .approach-heading h2,html[lang=zh-CN] .closing h2{font-size:35px}}
@media(max-width:430px){.site-header{padding-inline:20px;gap:12px}.brand-sub{max-width:185px;font-size:10px}.wordmark{font-size:29px}.header-actions{gap:9px}.languages{gap:5px}.menu-toggle{padding-inline:9px}.hero-title-block h1{font-size:38px}.hero-action-block{width:calc(100% - 26px)}.hero-action-block p{font-size:17px}.hero-motion-bar{left:16px;right:12px}.entry-links{grid-template-columns:1fr;gap:10px}.entry-links a{min-height:68px;padding:19px 22px;font-size:20px}.module-card h3{font-size:29px}.course-filters button{font-size:13px}.hero-motion-bar>span{max-width:18ch}html[lang=zh-CN] .hero-title-block h1{font-size:36px}}
@media(prefers-reduced-motion:reduce){*,*::before,*::after{transition:none!important;scroll-behavior:auto!important}}