/* ===== RESET & BASE ===== */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --clr-bg: #f4f7fb;
  --clr-surface: #ffffff;
  --clr-primary: #1a5276;
  --clr-primary-light: #2980b9;
  --clr-accent: #e67e22;
  --clr-accent-light: #f39c12;
  --clr-text: #2c3e50;
  --clr-text-light: #5d6d7e;
  --clr-border: #d5dbdb;
  --clr-math-bg: #eaf2f8;
  --clr-easy: #27ae60;
  --clr-medium: #e67e22;
  --clr-hard: #c0392b;
  --shadow: 0 2px 12px rgba(0,0,0,.08);
  --radius: 12px;
  --max-w: 920px;
}

html { scroll-behavior: smooth; font-size: 17px; }

body {
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--clr-bg);
  color: var(--clr-text);
  line-height: 1.7;
}

/* ===== LAYOUT ===== */
.container { max-width: var(--max-w); margin: 0 auto; padding: 0 1.2rem; }

/* ===== HEADER ===== */
header {
  background: linear-gradient(135deg, var(--clr-primary) 0%, var(--clr-primary-light) 100%);
  color: #fff;
  padding: 3rem 1.2rem 2.4rem;
  text-align: center;
}
header h1 { font-size: 2.4rem; font-weight: 800; letter-spacing: -.02em; margin-bottom: .3rem; }
header p { font-size: 1.1rem; opacity: .88; max-width: 600px; margin: 0 auto; }

/* ===== NAV ===== */
nav {
  position: sticky; top: 0; z-index: 100;
  background: var(--clr-surface);
  border-bottom: 1px solid var(--clr-border);
  box-shadow: 0 1px 6px rgba(0,0,0,.06);
}
nav ul {
  list-style: none; display: flex; flex-wrap: wrap;
  justify-content: center; gap: .2rem .6rem;
  padding: .7rem 1rem;
  max-width: var(--max-w); margin: 0 auto;
}
nav a {
  text-decoration: none; color: var(--clr-primary);
  font-weight: 600; font-size: .88rem;
  padding: .35rem .8rem; border-radius: 6px;
  transition: background .2s, color .2s;
}
nav a:hover { background: var(--clr-primary); color: #fff; }

/* ===== SECTIONS ===== */
section { padding: 2.5rem 0; }
section + section { border-top: 1px solid var(--clr-border); }

h2 {
  font-size: 1.7rem; font-weight: 700; color: var(--clr-primary);
  margin-bottom: 1.2rem; position: relative; padding-left: 1rem;
}
h2::before {
  content: ''; position: absolute; left: 0; top: .15em;
  width: 4px; height: 1.2em; background: var(--clr-accent); border-radius: 2px;
}
h3 { font-size: 1.15rem; color: var(--clr-primary-light); margin: 1.4rem 0 .6rem; }

/* ===== CARD ===== */
.card {
  background: var(--clr-surface);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.6rem 1.8rem;
  margin-bottom: 1.4rem;
}

/* ===== RULES LIST ===== */
.rules-list { list-style: none; counter-reset: rule; }
.rules-list li {
  counter-increment: rule;
  padding: .7rem 0 .7rem 2.6rem;
  position: relative;
  border-bottom: 1px dashed var(--clr-border);
}
.rules-list li:last-child { border-bottom: none; }
.rules-list li::before {
  content: counter(rule);
  position: absolute; left: 0; top: .65rem;
  width: 1.8rem; height: 1.8rem; border-radius: 50%;
  background: var(--clr-primary); color: #fff;
  font-weight: 700; font-size: .82rem;
  display: flex; align-items: center; justify-content: center;
}

/* ===== MATH DISPLAY ===== */
.math-block {
  background: var(--clr-math-bg);
  border-left: 4px solid var(--clr-primary-light);
  padding: 1rem 1.4rem;
  border-radius: 0 8px 8px 0;
  font-family: 'Cambria Math', 'Times New Roman', serif;
  font-size: 1.05rem;
  overflow-x: auto;
  margin: .8rem 0;
  line-height: 1.9;
}
.math-block .fraction {
  display: inline-block; text-align: center; vertical-align: middle;
}
.math-block .frac-num { display: block; border-bottom: 1px solid var(--clr-text); padding: 0 .3em; }
.math-block .frac-den { display: block; padding: 0 .3em; }

/* ===== IMAGE SECTION ===== */
.diagram-wrap {
  text-align: center;
  margin: 1.5rem 0;
}
.diagram-wrap img {
  max-width: 100%;
  height: auto;
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  border: 1px solid var(--clr-border);
}
.diagram-caption {
  font-size: .88rem; color: var(--clr-text-light);
  margin-top: .6rem; font-style: italic;
}

/* ===== EXAMPLE CARDS ===== */
.example {
  background: var(--clr-surface);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  margin-bottom: 1.6rem;
  overflow: hidden;
}
.example-header {
  background: linear-gradient(90deg, var(--clr-primary) 0%, var(--clr-primary-light) 100%);
  color: #fff; padding: .65rem 1.4rem;
  font-weight: 700; font-size: .95rem;
}
.example-body { padding: 1.2rem 1.6rem; }
.example-body .label { font-weight: 700; color: var(--clr-primary); margin-bottom: .2rem; }
.example-body .result-label { font-weight: 700; color: var(--clr-accent); margin-top: .6rem; }
.example-body .note { font-size: .9rem; color: var(--clr-text-light); margin-top: .3rem; }

/* ===== PRACTICE SECTION ===== */
.difficulty-badge {
  display: inline-block; padding: .2rem .7rem; border-radius: 20px;
  font-size: .78rem; font-weight: 700; color: #fff; margin-bottom: .5rem;
}
.difficulty-badge.easy   { background: var(--clr-easy); }
.difficulty-badge.medium { background: var(--clr-medium); }
.difficulty-badge.hard   { background: var(--clr-hard); }

.practice-item {
  background: var(--clr-surface);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.2rem 1.6rem;
  margin-bottom: 1rem;
}
.practice-item .math-block { margin-top: .5rem; }

/* ===== FOOTER ===== */
footer {
  background: var(--clr-primary);
  color: rgba(255,255,255,.75);
  text-align: center;
  padding: 1.6rem 1rem;
  font-size: .85rem;
  margin-top: 2rem;
}
footer strong { color: #fff; }

/* ===== RESPONSIVE ===== */
@media (max-width: 640px) {
  html { font-size: 15px; }
  header h1 { font-size: 1.7rem; }
  header { padding: 2rem 1rem 1.6rem; }
  .card { padding: 1.2rem; }
  .example-body { padding: 1rem; }
  nav ul { gap: .1rem .3rem; }
  nav a { font-size: .8rem; padding: .3rem .5rem; }
  h2 { font-size: 1.35rem; }
}