/* Class 12 Maths, Worked in Full — shared stylesheet */
:root{
  --bg:#fbfaf7; --paper:#ffffff; --ink:#1b2430; --ink-2:#56606d; --line:#e2ded6; --soft:#f3f1ec;
  --navy:#1f3a5f; --navy-soft:#eef3f9; --blue:#2f6aa3;
  --gold:#a8771f; --gold-soft:#faf4e6;
  --green:#2c7a4b; --green-soft:#ecf6f0;
  --rose:#ad3b37; --rose-soft:#fbefee;
  --violet:#5a47b5; --violet-soft:#f1eefc;
  --hl:#fff3c4;
  --serif:"Source Serif 4","Source Serif Pro",Georgia,"Times New Roman",serif;
  --sans:"Inter",-apple-system,BlinkMacSystemFont,"Segoe UI",system-ui,sans-serif;
  --mono:ui-monospace,"SF Mono",Menlo,Consolas,monospace;
  --radius:12px; --shadow:0 1px 2px rgba(20,30,50,.05),0 6px 20px rgba(20,30,50,.05);
}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]){
  --bg:#12161c; --paper:#181d25; --ink:#e7eaef; --ink-2:#a3acb9; --line:#2b323d; --soft:#1e242d;
  --navy:#9ec2ee; --navy-soft:#1a2533; --blue:#8bb8ea;
  --gold:#e3b862; --gold-soft:#2a2416; --green:#7fd3a0; --green-soft:#16271e;
  --rose:#f0928d; --rose-soft:#2c1a1a; --violet:#b9acff; --violet-soft:#211d36; --hl:#3a3418;
  --shadow:none; color-scheme:dark}}
:root[data-theme="dark"]{
  --bg:#12161c; --paper:#181d25; --ink:#e7eaef; --ink-2:#a3acb9; --line:#2b323d; --soft:#1e242d;
  --navy:#9ec2ee; --navy-soft:#1a2533; --blue:#8bb8ea;
  --gold:#e3b862; --gold-soft:#2a2416; --green:#7fd3a0; --green-soft:#16271e;
  --rose:#f0928d; --rose-soft:#2c1a1a; --violet:#b9acff; --violet-soft:#211d36; --hl:#3a3418;
  --shadow:none; color-scheme:dark}
*{box-sizing:border-box}
html{scroll-padding-top:72px}
body{margin:0;background:var(--bg);color:var(--ink);font-family:var(--serif);font-size:18px;line-height:1.65;-webkit-font-smoothing:antialiased}
a{color:var(--blue);text-underline-offset:3px}
a:hover{text-decoration-thickness:2px}
:focus-visible{outline:2px solid var(--blue);outline-offset:2px;border-radius:4px}
h1,h2,h3,h4{font-family:var(--sans);line-height:1.25;color:var(--navy);text-wrap:balance}
p,li{max-width:72ch}
.katex{font-size:1.06em;position:relative}
.nw{white-space:nowrap}   /* a short formula and the punctuation after it stay on one line */
.tw,.katex-display{position:relative}
.katex-display{overflow-x:auto;overflow-y:hidden;padding:4px 2px;margin:.7em 0}

/* ---------- top bar ---------- */
.top{position:sticky;top:0;z-index:20;background:color-mix(in srgb,var(--bg) 88%,transparent);backdrop-filter:blur(12px);-webkit-backdrop-filter:blur(12px);border-bottom:1px solid var(--line)}
.top .in{max-width:1180px;margin:0 auto;padding:10px 20px;display:flex;align-items:center;gap:16px;flex-wrap:wrap;font-family:var(--sans);font-size:14px}
.brand{font-weight:800;color:var(--navy);text-decoration:none;letter-spacing:-.01em}
.brand span{color:var(--gold)}
.crumbs{color:var(--ink-2);display:flex;gap:6px;flex-wrap:wrap;align-items:center;min-width:0}
.crumbs a{color:var(--ink-2);text-decoration:none}.crumbs a:hover{color:var(--ink)}
.top .grow{flex:1}
.toplinks{display:flex;gap:14px}.toplinks a{color:var(--ink-2);text-decoration:none;font-weight:600}.toplinks a:hover{color:var(--ink)}

/* ---------- layout ---------- */
.wrap{max-width:1180px;margin:0 auto;padding:0 20px}
.grid{display:grid;grid-template-columns:minmax(0,1fr) 300px;gap:40px;align-items:start}
@media (max-width:1000px){.grid{grid-template-columns:minmax(0,1fr)}.rail{display:none}}
main{min-width:0;padding-bottom:60px}
.rail{position:sticky;top:76px;display:grid;gap:18px;padding-top:34px;font-family:var(--sans);font-size:14px}
.rail .box{background:var(--paper);border:1px solid var(--line);border-radius:var(--radius);padding:14px 16px}
.rail h4{margin:0 0 8px;font-size:12px;letter-spacing:.08em;text-transform:uppercase;color:var(--ink-2)}
.rail ol{margin:0;padding-left:1.3em;display:grid;gap:4px}
.rail a{text-decoration:none;color:var(--ink)}.rail a:hover{color:var(--blue)}
.rail li.on>a{color:var(--blue);font-weight:700}

/* ad slots: reserved space, hidden until ads are switched on */
.ad{display:none}
body.ads .ad{display:flex;align-items:center;justify-content:center;border:1px dashed var(--line);border-radius:10px;color:var(--ink-2);font:12px var(--sans);background:var(--soft)}
body.ads .ad-top{min-height:90px;margin:18px 0}
body.ads .ad-mid{min-height:250px;margin:30px 0}
body.ads .ad-rail{min-height:600px}

/* ---------- problem page ---------- */
.ph{padding:34px 0 6px}
.kicker{font-family:var(--sans);font-size:13px;font-weight:700;letter-spacing:.06em;text-transform:uppercase;color:var(--gold)}
.ph h1{font-size:clamp(28px,4vw,40px);margin:.2em 0 .3em;letter-spacing:-.02em}
.ph .meta{font-family:var(--sans);font-size:14px;color:var(--ink-2);display:flex;gap:8px;flex-wrap:wrap}
.tag{display:inline-block;font:700 12px var(--sans);letter-spacing:.04em;text-transform:uppercase;padding:3px 9px;border-radius:999px;background:var(--navy-soft);color:var(--navy)}
.tag.t-ex{background:var(--violet-soft);color:var(--violet)}
.tag.t-misc{background:var(--gold-soft);color:var(--gold)}
.tag.t-mcq{background:var(--rose-soft);color:var(--rose)}

.box{border-radius:var(--radius);padding:16px 20px;margin:20px 0;border-left:5px solid var(--navy);background:var(--navy-soft)}
.box>:first-child{margin-top:0}.box>:last-child{margin-bottom:0}
.box .lbl{display:block;font:800 12px var(--sans);letter-spacing:.08em;text-transform:uppercase;margin-bottom:6px}
.q{background:var(--paper);border:1px solid var(--line);border-left:5px solid var(--navy);box-shadow:var(--shadow)}
.q .lbl{color:var(--navy)}
.idea{background:var(--rose-soft);border-left-color:var(--rose)}.idea .lbl{color:var(--rose)}
.gfa{background:var(--navy-soft)}.gfa .lbl{color:var(--navy)}
.gfa dl{display:grid;grid-template-columns:auto 1fr;gap:6px 16px;margin:0}
.gfa dt{font:700 14px var(--sans);color:var(--navy);padding-top:3px}
.gfa dd{margin:0}
.result{background:var(--green-soft);border-left-color:var(--green)}.result .lbl{color:var(--green)}
.take{background:var(--gold-soft);border-left-color:var(--gold)}.take .lbl{color:var(--gold)}
.pit{background:var(--rose-soft);border-left-color:var(--rose)}.pit .lbl{color:var(--rose)}
.check{background:var(--violet-soft);border-left-color:var(--violet)}.check .lbl{color:var(--violet)}
.play{background:var(--navy-soft);border-left-color:var(--navy)}.play .lbl{color:var(--navy)}
.take ol,.pit ol,.take ul,.pit ul{margin:.3em 0 0;padding-left:1.3em;display:grid;grid-template-columns:minmax(0,1fr);gap:6px}  /* minmax(0,…): a wide formula scrolls inside its item instead of widening the list */

h2.sec{font-size:22px;margin:42px 0 10px;padding-bottom:6px;border-bottom:2px solid var(--line)}

/* two views: exam answer | full explanation (no JavaScript: both show, exam answer first, no switch) */
.vswitch{display:inline-flex;gap:4px;padding:4px;margin:2px 0 4px;border-radius:999px;background:var(--soft);border:1px solid var(--line);font-family:var(--sans);max-width:100%}
.vswitch button{font:700 15px var(--sans);border:0;background:transparent;color:var(--ink-2);padding:9px 18px;border-radius:999px;cursor:pointer;white-space:nowrap}
.vswitch button[aria-pressed="true"]{background:var(--navy);color:var(--paper)}
@media (max-width:400px){.vswitch{display:flex}.vswitch button{flex:1;padding:9px 8px}}
html:not([data-view]) .vswitch{display:none}
[data-view="exam"] .v-full,[data-view="full"] .v-exam{display:none}
.exam{background:var(--paper);border:1px solid var(--line);border-left:5px solid var(--green);box-shadow:var(--shadow)}
.exam .lbl{color:var(--green)}
@media (max-width:640px){.exam{padding:14px 12px 14px 14px;border-left-width:4px}.exam .katex{font-size:1em}}
.exam p{margin:.55em 0}
.exam .katex-display>.katex{text-align:left;padding-left:1em}   /* written-answer layout: rows run down the left, as on an answer sheet */
[data-view="exam"] .rail .toc{display:none}                      /* the step list belongs to the full explanation */
.exam .ans{font-weight:700}
.exam .note{font:15px var(--sans);color:var(--ink-2);border-top:1px solid var(--line);padding-top:10px;margin-top:14px}
.vmore{font:15px var(--sans);color:var(--ink-2);margin:6px 0 0}
.vmore button{font:700 15px var(--sans);color:var(--blue);background:none;border:0;padding:0;cursor:pointer;text-decoration:underline;text-underline-offset:3px}

/* basics needed */
.needs{display:grid;gap:10px;margin:14px 0}
.need{display:grid;grid-template-columns:auto 1fr;gap:4px 14px;background:var(--paper);border:1px solid var(--line);border-radius:10px;padding:12px 16px}
.need .n{font:800 13px var(--sans);color:var(--paper);background:var(--navy);width:24px;height:24px;border-radius:50%;display:grid;place-items:center;grid-row:span 2;margin-top:2px}
.need>:not(.n){grid-column:2;min-width:0}  /* display maths or extra paragraphs never fall into the number column */
.need b{font-family:var(--sans);font-size:15px;color:var(--ink)}
.need p{margin:0;font-size:16.5px}
.need .go{font:600 13px var(--sans)}

/* steps */
ol.steps{list-style:none;margin:0;padding:0;counter-reset:st}
ol.steps>li{counter-increment:st;position:relative;padding:4px 0 8px 84px;margin:0 0 26px}
ol.steps>li::before{content:"Step " counter(st);position:absolute;left:0;top:6px;font:800 14px var(--sans);color:var(--blue);white-space:nowrap}
@media (max-width:640px){ol.steps>li{padding-left:0}ol.steps>li::before{position:static;display:block;margin-bottom:2px}}
ol.steps h3{margin:0 0 6px;font-size:19px}
.why,.basic{font-size:16.5px;border-radius:10px;padding:10px 14px;margin:10px 0}
.why{background:var(--soft)}
.why::before{content:"Why: ";font:800 13px var(--sans);color:var(--ink-2);letter-spacing:.04em;text-transform:uppercase}
.basic{background:var(--navy-soft)}
.basic::before{content:"Basic behind it: ";font:800 13px var(--sans);color:var(--navy);letter-spacing:.04em;text-transform:uppercase}
.boxed{display:inline-block;border:1.5px solid currentColor;padding:1px 8px;border-radius:4px}
.hl{background:var(--hl);border-radius:3px;padding:0 3px}

/* figures */
figure.fig{margin:22px 0;background:var(--paper);border:1px solid var(--line);border-radius:var(--radius);padding:14px}
figure.fig svg{display:block;width:100%;height:auto;max-width:640px;margin:0 auto}
figure.fig figcaption{font-size:15px;color:var(--ink-2);font-style:italic;text-align:center;margin-top:8px;max-width:none}
figure.fig svg text{font-family:var(--sans)}

/* tables */
.tw{overflow-x:auto;margin:16px 0}
table.t{border-collapse:collapse;width:100%;font-size:16px;background:var(--paper)}
table.t th,table.t td{padding:8px 12px;border-bottom:1px solid var(--line);text-align:left;vertical-align:top}
table.t th{font:700 13px var(--sans);letter-spacing:.04em;color:var(--ink-2);border-bottom:2px solid var(--line)}
table.t tr:first-child th{text-transform:uppercase}
table.t th .katex{text-transform:none;font-weight:400}
table.t tbody th,table.t tr+tr th{border-bottom:1px solid var(--line);white-space:nowrap}
table.t td.c,table.t th.c{text-align:center}

/* prev / next */
.pn{display:grid;grid-template-columns:1fr 1fr;gap:14px;margin:46px 0 10px;font-family:var(--sans)}
.pn a{display:block;background:var(--paper);border:1px solid var(--line);border-radius:var(--radius);padding:14px 16px;text-decoration:none;color:var(--ink)}
.pn a:hover{border-color:var(--blue)}
.pn small{display:block;color:var(--ink-2);font-size:12px;font-weight:700;letter-spacing:.06em;text-transform:uppercase}
.pn .nx{text-align:right;grid-column:2}

/* ---------- index pages ---------- */
.hero{padding:46px 0 10px}
.hero h1{font-size:clamp(32px,5vw,52px);margin:.1em 0 .2em;letter-spacing:-.025em}
.hero p{font-size:20px;color:var(--ink-2);max-width:62ch;margin:0}
.chgrid{display:grid;grid-template-columns:repeat(auto-fill,minmax(min(100%,300px),1fr));gap:14px;margin:28px 0 60px}
.chcard{display:block;background:var(--paper);border:1px solid var(--line);border-radius:var(--radius);padding:18px;text-decoration:none;color:var(--ink);font-family:var(--sans)}
.chcard:hover{border-color:var(--blue)}
.chcard .no{font-size:12px;font-weight:800;letter-spacing:.08em;text-transform:uppercase;color:var(--gold)}
.chcard h3{margin:4px 0 6px;font-size:19px}
.chcard p{margin:0;font-size:13.5px;color:var(--ink-2)}
.chcard.soon{opacity:.6}
.set{margin:30px 0}
.set h2{font-size:21px;margin:0 0 10px}
.plist{list-style:none;margin:0;padding:0;display:grid;gap:8px}
.plist a,.plist span.x{display:grid;grid-template-columns:150px 1fr auto;gap:12px;align-items:baseline;background:var(--paper);border:1px solid var(--line);border-radius:10px;padding:11px 16px;text-decoration:none;color:var(--ink);font-family:var(--sans);font-size:15px}
.plist a:hover{border-color:var(--blue)}
.plist .pid{font-weight:800;color:var(--navy)}
.plist .st{font-size:12px;color:var(--ink-2)}
.plist span.x{opacity:.55}
@media (max-width:560px){.plist a,.plist span.x{grid-template-columns:1fr}}
.lessonlinks{display:flex;gap:10px;flex-wrap:wrap;margin:18px 0}
.btn{display:inline-block;font:700 14px var(--sans);text-decoration:none;padding:9px 16px;border-radius:999px;background:var(--navy);color:var(--paper)}
.btn.alt{background:var(--paper);color:var(--navy);border:1px solid var(--line)}
footer.foot{border-top:1px solid var(--line);padding:24px 0 40px;font:13px var(--sans);color:var(--ink-2)}
@media print{.top,.rail,.ad,.pn{display:none}.grid{display:block}}

/* phones: keep figure text readable (>= ~11px) by letting wide figures scroll inside their frame */
@media (max-width:640px){figure.fig{overflow-x:auto}figure.fig svg{min-width:520px}}
/* KaTeX draws root signs and wide delimiters with small inline SVGs; give them back KaTeX's own sizing inside figures */
figure.fig .katex svg{display:block;position:absolute;width:100%;height:inherit;min-width:0;max-width:none;margin:0}

/* ---------- past papers: two views ("With chapter links" / "Answers only") and the chapter connections ---------- */
html[data-pp] .vswitch.ppswitch{display:inline-flex;margin-top:12px}
@media (max-width:400px){html[data-pp] .vswitch.ppswitch{display:flex}}
[data-pp="plain"] .pp-c{display:none!important}
.tag.t-pp{background:var(--gold-soft);color:var(--gold)}
.ph .meta{display:flex;flex-wrap:wrap;gap:6px 8px;align-items:center}
a.tag.t-ch{background:var(--violet-soft);color:var(--violet);text-decoration:none;text-transform:none;letter-spacing:0;font-weight:600}
a.tag.t-ch:hover{text-decoration:underline}
.box.from .lbl{color:var(--violet)}
.box.from p{margin:.35em 0}
.box.from .links,.hero .links{font-family:var(--sans);font-size:15px}
.box.from .also{font-size:15px;color:var(--ink-2)}
.qtype{margin:.5em 0 .6em}
.qtype .qt{margin:.35em 0 .2em}
.qtype .qt a{font-weight:700}
.qtype .qt-h{margin-top:.6em}
.qtype .qt-h:first-child{margin-top:0}
details.qtspot{margin:.1em 0 .3em}
details.q-pp{margin:10px 0 0}
.q-pp .why-b{font-family:var(--sans);font-size:14.5px;line-height:1.7;color:var(--ink-2)}
.q-pp a{white-space:nowrap}
.chpath{margin-top:6px}
.chpath .pn{margin-top:8px}
.chpath-h{font-family:var(--sans);font-size:14px;color:var(--ink-2);margin:26px 0 0}
.plist .topic{color:var(--ink-2);font-size:13px}
.hero .ways{display:flex;flex-wrap:wrap;gap:10px}
.box.q .or{font-weight:700;text-align:center;color:var(--ink-2);margin:.4em 0}
.pp-note{background:var(--gold-soft);border-radius:10px;padding:8px 14px;font-size:15px}

/* ---------- past papers: the paper at a glance (fact chips + section table) ---------- */
.ppfacts{list-style:none;margin:14px 0 4px;padding:0;display:flex;flex-wrap:wrap;gap:8px;font-family:var(--sans)}
.ppfacts li{background:var(--soft);border:1px solid var(--line);border-radius:999px;padding:6px 14px;font-size:15px;color:var(--ink-2)}
.ppfacts b{color:var(--ink);font-weight:800;font-variant-numeric:tabular-nums}
.ppformat h2{margin-bottom:10px}
.pptab-wrap{overflow-x:auto}
.pptab{width:100%;border-collapse:collapse;font-family:var(--sans);font-size:15px;line-height:1.4}
.pptab th,.pptab td{text-align:left;vertical-align:top;padding:10px 12px;border-bottom:1px solid var(--line)}
.pptab thead th{font-size:12px;letter-spacing:.06em;text-transform:uppercase;color:var(--ink-2);font-weight:700}
.pptab tbody th{font-weight:700;color:var(--ink);white-space:nowrap}
.pptab td{color:var(--ink-2)}
.pptab .num{text-align:right;white-space:nowrap;font-variant-numeric:tabular-nums;color:var(--ink)}
.pptab tr.tot th,.pptab tr.tot td{border-bottom:0;font-weight:800;color:var(--ink)}
/* phones: each section becomes a two-line card (section and marks, then questions and type) */
@media (max-width:560px){
  .pptab thead{display:none}
  .pptab,.pptab tbody{display:block}
  .pptab tr{display:grid;grid-template-columns:1fr auto;column-gap:12px;padding:10px 2px;border-bottom:1px solid var(--line)}
  .pptab th,.pptab td{display:block;padding:0;border:0}
  .pptab tbody th{grid-column:1;grid-row:1}
  .pptab .num{grid-column:2;grid-row:1}
  .pptab td.q{grid-column:1 / -1;grid-row:2;font-size:14px;margin-top:2px}
  .pptab td.t{grid-column:1 / -1;grid-row:3;font-size:14px}
  .pptab td.t:empty{display:none}
  .pptab tr.tot{border-bottom:0}
}
.box.ppbox .lbl{background:var(--gold-soft);color:var(--gold)}

/* ---------- past papers: closed "Why?" explanations under the steps of an exam answer ---------- */
.box.exam .exhead{display:flex;flex-wrap:wrap;align-items:center;justify-content:space-between;gap:8px;margin-bottom:6px}
.box.exam .exhead .lbl{margin:0}
.whyall{font:600 13px var(--sans);border:1px solid var(--line);background:var(--paper);color:var(--ink-2);border-radius:999px;padding:5px 12px;cursor:pointer}
.whyall:hover{color:var(--ink)}
details.stepwhy{margin:.15em 0 .7em}
details.stepwhy>summary{display:inline-block;list-style:none;cursor:pointer;font:600 12.5px var(--sans);letter-spacing:.02em;color:var(--violet);background:var(--violet-soft);border-radius:999px;padding:2px 10px;user-select:none}
details.stepwhy>summary::-webkit-details-marker{display:none}
details.stepwhy>summary::after{content:" ▸"}
details.stepwhy[open]>summary::after{content:" ▾"}
details.stepwhy>summary:focus-visible{outline:2px solid var(--violet);outline-offset:2px}
.why-b{margin-top:8px;padding:10px 14px;border-left:3px solid var(--violet);background:var(--soft);border-radius:0 10px 10px 0;font-size:15.5px;line-height:1.55;color:var(--ink)}
.why-b p{margin:.35em 0}
@media print{details.stepwhy .why-b{display:block}}
.exam .why-b .katex-display>.katex{padding-left:.25em}   /* the step-by-step working in a "Why?" needs the width more than the answer-sheet indent */
@media (max-width:420px){.why-b{padding:10px 10px}.why-b .katex-display{font-size:.94em}}