A. How to use the first derivative test? [Use a graphing calculator to see the graph.] 0 x 2 Example: f ( x) cos x 2 sin x, 2 Step 1: f is a continuous function. Step 2: Find all critical numbers of f. f ' ( x) 2 cos x( sin x) 2 cos x 2 cos x(sin x 1) 0 cos x = 0 ==> x = π/2, 3π/2 sin x = -1 ==> x = 3π/2 Step 3: We have 3 intervals. 0 < x < π/2, π/2 < x < 3π/2, 3π/2 < x < 2π 0 < x < π/2 π/2 < x < 3π/2 3π/2 < x < 2π Interval − ' Sign of f ( x) f(x) decreasing + − increasing decreasing ' At x = π/2 : f changes from negative to positive, then f has a local minimum at π/2. The local minimum value is f ( / 2) cos ( / 2) 2 sin( / 2) 2 2 ' At x = 3π/2 : f changes from positive to negative, then f has a local maximum at 3π/2. The local maximum value is f (3 / 2) cos (3 / 2) 2 sin(3 / 2) 2 2 1 B. How can we find intervals of concavity and inflection points? f '' ( x) 2 sin x(1 sin x) 2 cos2 x '' Step 1 : Find f ( x). Step 2 : Solve for x, f ( x) 0 . '' Using trig. identity, we get 2 sin x sin x 1 0 2 Factoring: (2 sin x 1)(sin x 1) 0 1. sin x = ½ , x = π/6, 5π/6. 2. sin x = -1 , x = 3π/2. Interval Sign of f '' 0 < x < π/6 π/6 < x < 5π/6 5π/6 < x < 3π/2 3π/2 < x < 2π − + − − CD Inflection points: CU CD CD 1 5 1 ( , ), ( , ) 6 4 6 4 1 , 4 (i) x = π/6, f ( / 6) cos ( / 6) 2 sin( / 6) (ii) x = 5π/6, f (5 / 6) cos (5 / 6) 2 sin( 5 / 6) 2 2 1 . 4 C. How to use the second derivative test? Step 1 : Find all critical numbers. x = π/2, 3π/2. '' Step 2: Find f ( x). f '' ( x) 2 sin x(1 sin x) 2 cos2 x 2(sin 2 x cos2 x sin x) '' Step 3: Evaluate f ( x) at x = π/2, 3π/2. f '' ( / 2) 4 0 , then f has a local minimum at x = π/2. f '' (3 / 2) 0 , we don’t get any information from the second derivative test. We need to use the first derivative test. 2