Sal justifies the second derivative test, which is a way of determining relative minima & maxima, and gives an example. Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/ap-calculus-ab/ab-derivatives-analyze-functions/ab-justification-with-second-derivative/e/second-derivative-test?utm_source=YT&utm_medium=Desc&utm_campaign=APCalculusAB Watch the next lesson: https://www.khanacademy.org/math/ap-calculus-ab/ab-derivatives-analyze-functions/ab-inflection-points/v/analyzing-concavity-algebraically?utm_source=YT&utm_medium=Desc&utm_campaign=APCalculusAB Missed the previous lesson? https://www.khanacademy.org/math/ap-calculus-ab/ab-derivatives-analyze-functions/ab-justification-with-second-derivative/v/inflection-points-from-second-derivative?utm_source=YT&utm_medium=Desc&utm_campaign=APCalculusAB AP Calculus AB on Khan Academy: Bill Scott uses Khan Academy to teach AP Calculus at…