:root {
  --ink: #2b3a55;
  --muted: #6b7a99;
  --sky: #4fb0ff;
  --sky-dark: #2f8fe0;
  --orange: #ff8a3d;
  --pink: #ff5c8a;
  --green: #3cbf7c;
  --purple: #8e6cf0;
  --yellow: #ffd24d;
  --cream: #fff9ec;
  --card: #ffffff;
  --line: #e4e9f2;
  --radius: 20px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: "Zen Maru Gothic", "Hiragino Maru Gothic ProN", "Hiragino Sans", "Yu Gothic", sans-serif;
  font-weight: 500;
  color: var(--ink);
  background: linear-gradient(180deg, var(--cream) 0%, #eef6ff 100%);
  line-height: 1.8;
  font-size: 17px;
}

ruby rt { font-size: 0.5em; color: var(--muted); font-weight: 500; }

b { font-weight: 900; }
.pink { color: var(--pink); }
.green { color: #2f9f5a; }
.orange { color: var(--orange); }
.purple { color: var(--purple); }

/* ---------- hero ---------- */
.hero {
  text-align: center;
  padding: 48px 16px 28px;
  background:
    radial-gradient(circle at 12% 20%, rgba(255,210,77,0.35), transparent 40%),
    radial-gradient(circle at 88% 10%, rgba(79,176,255,0.35), transparent 40%),
    radial-gradient(circle at 80% 90%, rgba(255,138,61,0.25), transparent 40%);
}
.eyebrow {
  display: inline-block;
  margin: 0 0 8px;
  padding: 4px 16px;
  background: var(--ink);
  color: #fff;
  border-radius: 999px;
  font-size: 14px;
  letter-spacing: 0.1em;
}
.hero h1 {
  margin: 8px 0;
  font-size: clamp(30px, 6vw, 52px);
  font-weight: 900;
  line-height: 1.4;
}
.lead { margin: 8px auto 20px; max-width: 640px; color: var(--muted); }
.toc { display: flex; flex-wrap: wrap; gap: 10px; justify-content: center; }
.toc a {
  text-decoration: none;
  color: var(--ink);
  background: #fff;
  padding: 8px 18px;
  border-radius: 999px;
  font-weight: 700;
  box-shadow: 0 3px 0 var(--line);
  transition: transform .15s;
}
.toc a:hover { transform: translateY(-2px); }

/* ---------- sections ---------- */
main { max-width: 960px; margin: 0 auto; padding: 0 16px 60px; }
.section { margin: 40px 0; }
.section h2 {
  display: flex; align-items: center; gap: 12px;
  font-size: clamp(22px, 4vw, 30px);
  font-weight: 900;
  margin: 0 0 8px;
}
.num {
  display: inline-grid; place-items: center;
  width: 40px; height: 40px; border-radius: 50%;
  background: var(--orange); color: #fff; font-size: 20px;
  box-shadow: 0 4px 0 #d96a22;
}
.note { color: var(--muted); margin: 0 0 16px; font-size: 15px; }

.card {
  background: var(--card);
  border-radius: var(--radius);
  padding: 20px;
  margin: 18px 0;
  box-shadow: 0 6px 0 var(--line);
}
.card h3 {
  margin: 0 0 12px;
  font-size: clamp(18px, 3vw, 22px);
  font-weight: 900;
  display: flex; align-items: center; gap: 10px; flex-wrap: wrap;
}
.step {
  background: var(--sky); color: #fff;
  padding: 2px 12px; border-radius: 999px; font-size: 14px;
}
.card-body { display: grid; grid-template-columns: 1fr 1fr; gap: 20px; align-items: center; }
.fig { width: 100%; height: auto; font-family: inherit; }
.text p { margin: 6px 0; }
.text ul, .text ol { margin: 6px 0; padding-left: 22px; }
.text .big { font-size: 24px; font-weight: 900; margin: 10px 0; }
.text .sub { font-size: 14px; color: var(--muted); background: #f6f8fc; padding: 10px 12px; border-radius: 12px; }
.formula-line {
  background: #fff5ea; border: 2px dashed var(--orange);
  padding: 8px 14px; border-radius: 12px; font-weight: 900; font-size: 19px;
  display: inline-block;
}

/* 分数 */
.frac {
  display: inline-flex; flex-direction: column; align-items: center;
  vertical-align: middle; line-height: 1.1; font-size: 0.85em; margin: 0 2px;
}
.frac span:first-child { border-bottom: 2px solid currentColor; padding: 0 4px; }
.frac span:last-child { padding: 0 4px; }

/* ---------- derive ---------- */
.derive { display: flex; flex-direction: column; gap: 6px; margin: 8px 0 16px; }
.derive .row { display: flex; align-items: center; flex-wrap: wrap; gap: 6px; }
.derive .row.gap { margin-top: 14px; }
.lbl { min-width: 110px; font-weight: 900; }
.eq { font-weight: 900; }
.op { font-weight: 900; color: var(--muted); }
.chip {
  display: inline-block; padding: 2px 12px; border-radius: 10px;
  font-weight: 900; color: #fff; background: var(--ink);
}
.chip.pink { background: var(--pink); }
.chip.green { background: var(--green); }
.chip.orange { background: var(--orange); }
.chip.purple { background: var(--purple); }
.chip.ink { background: #eef2f8; color: var(--ink); }
.why { font-size: 14px; color: var(--muted); margin: 8px 0 0; }

.formula-box {
  background: linear-gradient(135deg, #fff5ea, #f3edff);
  border: 3px solid var(--purple);
  border-radius: 16px;
  padding: 14px 18px;
  text-align: center;
}
.fb-title { margin: 0; font-size: 14px; font-weight: 900; color: var(--purple); letter-spacing: .1em; }
.fb-main { margin: 6px 0; font-size: clamp(18px, 3.4vw, 26px); font-weight: 900; }
.hl { padding: 0 6px; border-radius: 8px; background: #fff; box-shadow: inset 0 -6px 0 rgba(79,176,255,.35); }
.hl.purple { box-shadow: inset 0 -6px 0 rgba(142,108,240,.35); }
.hl.orange { box-shadow: inset 0 -6px 0 rgba(255,138,61,.4); }
.fb-sub { margin: 4px 0 0; font-size: 14px; color: var(--muted); }
.math { font-family: "Times New Roman", serif; font-style: italic; font-size: 18px; color: var(--ink); }

/* ---------- player ---------- */
.player {
  background: var(--card);
  border-radius: var(--radius);
  padding: 14px;
  box-shadow: 0 6px 0 var(--line);
}
.screen { position: relative; border-radius: 14px; overflow: hidden; background: #fff9ec; }
#stage { display: block; width: 100%; height: auto; aspect-ratio: 16 / 9; }
.big-play {
  position: absolute; inset: 0; margin: auto;
  width: 130px; height: 130px; border-radius: 50%;
  border: none; background: rgba(255,138,61,0.95); color: #fff;
  font-size: 44px; cursor: pointer;
  display: flex; flex-direction: column; align-items: center; justify-content: center;
  box-shadow: 0 8px 0 #d96a22, 0 0 0 12px rgba(255,255,255,.6);
  transition: transform .15s;
  font-family: inherit;
}
.big-play small { font-size: 15px; font-weight: 900; margin-top: -4px; }
.big-play:hover { transform: scale(1.06); }
.big-play.hidden { display: none; }
.caption {
  min-height: 3.2em;
  margin: 12px 4px 8px;
  padding: 10px 16px;
  background: #f3f7ff;
  border-left: 6px solid var(--sky);
  border-radius: 10px;
  font-size: 16px;
}
.controls { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin: 8px 0; }
.btn {
  font-family: inherit; font-weight: 900; font-size: 15px;
  padding: 8px 14px; border-radius: 999px; border: none; cursor: pointer;
  background: #eef2f8; color: var(--ink);
  box-shadow: 0 3px 0 #d8dfeb;
}
.btn:active { transform: translateY(2px); box-shadow: none; }
.btn.primary { background: var(--orange); color: #fff; box-shadow: 0 3px 0 #d96a22; }
.toggle { margin-left: auto; font-weight: 700; font-size: 15px; display: flex; align-items: center; gap: 4px; cursor: pointer; }
.toggle input { width: 18px; height: 18px; accent-color: var(--orange); }
.seek { display: flex; align-items: center; gap: 12px; margin: 4px 0 10px; }
.seek input[type=range] { flex: 1; accent-color: var(--orange); }
.seek span { font-variant-numeric: tabular-nums; font-weight: 700; color: var(--muted); font-size: 14px; }
.chapters { list-style: none; margin: 0; padding: 0; display: flex; flex-wrap: wrap; gap: 6px; }
.chapters button {
  font-family: inherit; font-size: 13px; font-weight: 700;
  padding: 4px 12px; border-radius: 999px; border: 2px solid var(--line);
  background: #fff; color: var(--ink); cursor: pointer;
}
.chapters button.active { background: var(--sky); border-color: var(--sky); color: #fff; }

/* ---------- calc ---------- */
.calc {
  background: var(--card); border-radius: var(--radius); padding: 20px;
  box-shadow: 0 6px 0 var(--line);
  display: grid; grid-template-columns: 300px 1fr; gap: 20px;
}
.radius-label { font-weight: 900; font-size: 20px; display: flex; align-items: center; gap: 8px; }
.radius-label input {
  font-family: inherit; font-size: 24px; font-weight: 900; width: 120px;
  padding: 6px 10px; border-radius: 12px; border: 3px solid var(--sky); color: var(--ink);
  text-align: right;
}
.presets { display: flex; flex-wrap: wrap; gap: 6px; margin: 12px 0; }
.presets button {
  font-family: inherit; font-size: 13px; font-weight: 700;
  padding: 4px 10px; border-radius: 999px; border: none; cursor: pointer;
  background: #fff1e6; color: #b7541b;
}
.presets button:hover { background: var(--orange); color: #fff; }
.ball-preview { width: 100%; max-width: 260px; display: block; margin: 0 auto; font-family: inherit; }
.calc-steps { margin: 0; padding-left: 26px; font-size: 18px; }
.calc-steps li { margin: 6px 0; }
.calc-steps .v { font-weight: 900; color: var(--sky-dark); }
.calc-steps .k { color: var(--muted); font-size: 14px; }
.result {
  margin-top: 14px; padding: 14px 18px;
  background: linear-gradient(135deg, #fff5ea, #fff); border: 3px solid var(--orange);
  border-radius: 16px;
}
.result .ans { font-size: clamp(22px, 4vw, 30px); font-weight: 900; margin: 0; }
.result .cmp { margin: 4px 0 0; color: var(--muted); font-size: 15px; }

/* ---------- try ---------- */
.howto li { margin: 6px 0; }
.mini-fact {
  background: #fff8dc; border-radius: 14px; padding: 14px 16px; font-size: 15px;
}
.mini-fact p { margin: 4px 0; }
.mf-title { font-weight: 900; font-size: 16px; }

.footer { text-align: center; color: var(--muted); font-size: 14px; padding: 20px; }

@media (max-width: 720px) {
  body { font-size: 16px; }
  .card-body, .calc { grid-template-columns: 1fr; }
  .lbl { min-width: 0; }
  .big-play { width: 100px; height: 100px; font-size: 34px; }
  .toggle { margin-left: 0; }
}

/* ---------- slicer ---------- */
.slicer { display: grid; gap: 10px; }
.slider-label { display: flex; align-items: center; gap: 12px; font-weight: 900; font-size: 18px; }
.slider-label input[type=range] { flex: 1; accent-color: var(--orange); height: 28px; }
.text .tool { background: #f6f8fc; padding: 8px 12px; border-radius: 12px; }
.why-inline { font-size: 13px; color: var(--muted); }