Pedagogy: refresh aging embedded YouTube videos (audit)

## Why · 이유

Several notebooks embed YouTube videos as supplementary explanations. Most are foundational math from Khan Academy, MIT OCW (Strang), or 3Blue1Brown — pedagogically timeless and worth keeping. A handful, however, are old enough or off-topic enough that a refresh would help: dated software UIs or industry talks whose tooling landscape has moved on.

This issue tracks the audit and the refresh candidates. KEEP entries are listed at the bottom for reference; no action needed for those.

수치해법 노트북에 삽입된 YouTube 영상 중 일부는 화면이 오래되었으므로 새 영상으로 교체 후보를 정리.

## Refresh candidates · 교체 후보

- [ ] **`20_probability/30_pandas.ipynb`** (cell 166, right after a `sns.pairplot` example) — Wes McKinney *Data science without borders* (https://www.youtube.com/watch?v=wdmf1msbtVs, 2017). Industry/tooling perspective; the Arrow/pandas ecosystem has moved meaningfully since 2017. Either drop or replace with a more recent overview.
- [ ] **`15_optimization/030_Classification_Optimization.ipynb`** — sigmoid (https://youtu.be/PIjno6paszY?t=650) and cross-entropy (https://youtu.be/6vzchGYEJBc) embeds. Source/upload metadata could not be confirmed during audit. ML pedagogy has improved a lot in the last few years (StatQuest, Andrej Karpathy zero-to-hero, 3Blue1Brown NN series) — strong replacement pool.
- [ ] **`tutorial/10.git.ipynb`** (cell 182) — Harvard CS50 EP. 4 git/GitHub lecture (https://www.youtube.com/watch?v=dAHgwd2U0Jg, 2018). UI elements (GitHub web, terminal output) are 7+ years old; modern git defaults (`main`, `switch`/`restore`) post-date the lecture. Coordinate with #418 which already refreshes the notebook's prose.
- [ ] **`tutorial/10.git.ipynb`** (cell 154) — sync-upstream playlist link (https://www.youtube.com/watch?v=P39pzSQx5rY&list=PLA6B0Lmr9oJNDafh3ndnmbXv0I9wddv63). Verify the playlist still resolves and is current; otherwise replace with a single up-to-date clip.
- [ ] **`20_probability/15_random_variables.ipynb`** — Khan Academy *How to create a histogram* (https://www.youtube.com/watch?v=gSEYtAjuZ-Y). Elementary-grade level; statistics-track histogram explainers may be a better fit. Lower priority.

## Not a refresh candidate · 교체 대상 아님

- **`tutorial/05.jupyter_notebook.ipynb`** (cell 11) — Ray Dalio clip (https://youtu.be/xguam0TKMw8). Intentional: the surrounding cell illustrates Jupyter's flexibility to embed arbitrary media (videos, images, etc.). The specific clip is unrelated to numerical methods *by design* — it shows that the notebook can host any web content. Keep as-is.

## KEEP — no action (audit record) · 유지 — 조치 불필요

Timeless / well-presented embeds confirmed during this audit

| Notebook | Video | Source |
|---|---|---|
| `00_introduction/50_pandas.ipynb` | pandas in 10 minutes (Wes McKinney, 2019) | https://www.youtube.com/watch?v=_T8LGqJtuGc |
| `20_probability/10_probability.ipynb` | Dice / independent events; Venn diagrams; *Hardest problem on hardest test* (3Blue1Brown) | Khan Academy ×2, 3Blue1Brown |
| `20_probability/20_statistics.ipynb` | Residuals & least-squares regression | Khan Academy |
| `40_linear_algebra_1/10_vector_addition_subtraction.ipynb` | Vector intro & addition | Khan Academy |
| `40_linear_algebra_1/20_vector_dot_cross_product.ipynb` | Dot product & cross product intro | Khan Academy |
| `40_linear_algebra_1/40_matrix_and_vector.ipynb` | MIT 18.06 L3 — Matrix Multiplication (Strang) | MIT OCW |
| `60_linear_algebra_2/100_Systems_of_Linear_Equations.ipynb` | MIT 18.06 L2 — Gauss Elimination (Strang) | MIT OCW |
| `60_linear_algebra_2/150_Inverse_matrix.ipynb` | MIT 18.06 L3 — Inverse Matrices (Strang); KA 3×3 inverse | MIT OCW, Khan Academy |
| `60_linear_algebra_2/200_Eigenvalues_of_a_Matrix_PowerMethod.ipynb` | Eigenvectors and eigenvalues (3Blue1Brown, 2016) | https://www.youtube.com/watch?v=PFDu9oVAE-g |
| `10_root_finding/45_newton_raphson_complex.ipynb` | Newton's Fractal (3Blue1Brown) | https://youtu.be/-RdOwhmqP5s |
| `30_num_int/10_first_order.ipynb` | Trapezoidal sums | Khan Academy |
| `50_ode/10_Forward_Euler.ipynb` | Euler's method | Khan Academy |
| `50_ode/55_Spread_of_disease.ipynb` | SIR model visualization (3Blue1Brown) | https://youtu.be/gxAaO2rsdIs |
| `tutorial/10.git.ipynb` (cell 183) | Scott Chacon @ FOSDEM 2024 — modern git features | https://www.youtube.com/watch?v=aolI_Rz0ZqY |

Khan Academy explanations are pedagogically canonical and resolution-independent; Strang's MIT 18.06 lectures are reference material; 3Blue1Brown's visualization quality stays evergreen.

## Approach · 접근

Treat each checkbox as an independent PR — small, focused, easy to review. For each replacement, prefer (a) clearer pedagogy, (b) current UI/tooling where relevant, (c) Creative Commons / educational channels we already cite (Khan Academy, MIT OCW, 3Blue1Brown, StatQuest). If a strong replacement isn't found, removing the embed and strengthening the prose explanation is also acceptable.