MATH U101 · Narrated walkthrough · Module 6 §14.7
Critical points and the second-derivative test
From a flat spot to max, min or saddle, built from scratch, then Tutorial 7 Q5 and Q6 solved. Press play, keep the sound on, and answer the four questions it stops to ask you.
About seven minutes, watch once. The narration stops four times to ask what the next move is. Commit to an answer before it continues: those pauses are the point, because they practise starting a question, not following one. Captions are on by default; the full transcript is below the player.
Redo Q5 on paper without looking, then check it against the tutorial companion (Q6 is just below it). The test and its D = 0 cases are written up in Module 6 notes §2; the slow version with surfaces you can explore is the Module 6 lesson.