.video-wrapper { position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden; max-width: 100%; margin: 1.5rem 0; border-radius: 12px; }
.video-wrapper iframe { position: absolute; top: 0; left: 0; width: 100%; height: 100%; border: 0; }
.exercise { background: #161616; border-left: 3px solid #444; padding: 1rem 1.2rem; margin: 1rem 0; border-radius: 0 8px 8px 0; }
.exercise summary { cursor: pointer; color: #aaa; font-weight: 600; }
.exercise summary:hover { color: #fff; }
.exercise p { margin: 0.5rem 0; }
.exercise details { margin-top: 0.8rem; }
.exercise details summary { color: #888; font-weight: 500; font-size: 0.9rem; }
.exercise .solution { margin-top: 0.5rem; padding: 0.8rem; background: #1a1a1a; border-radius: 6px; }
.formula-box { background: #161616; border: 1px solid #333; padding: 1rem; border-radius: 8px; margin: 1rem 0; text-align: center; font-size: 1.1rem; color: #ddd; }
.formula-box .label { font-size: 0.82rem; color: #666; margin-bottom: 0.5rem; }
.formula-box .formula { font-size: 1.2rem; color: #fff; }