Max/Min Problems

Lecture 11 18.01 Fall 2006 Lecture 11: Max/Min Problems Example 1. y = ln x (same function as in last lecture) x 1/e x0=e Figure 1: Graph of y = • What is the maximum value? Answer: y = ln x . x 1 . e • Where (or at what point) is the maximum achieved? Answer: x = e. (See Fig. 1).) Beware: Some people will ask “What is the maximum?”. The answer is not e. You will get so used to finding the critical point x = e, the main calculus step, that you will forget to find the maximum 1 1 value y = . Both the critical point x = e and critical value y = are important. Together, they e e 1 form the point of the graph (e, ) where it turns around. e Example 2. Find the max and the min of the function in Fig. 2 Answer: If you’ve already graphed the function, it’s obvious where the maximum and minimum values are. The point is to find the maximum and minimum without sketching the whole graph. Idea: Look for the max and min among the critical points and endpoints.You can see from Fig. 2 that we only need to compare the heights or y-values corresponding to endpoints and critical points. (Watch out for discontinuities!) 1 Lecture 11 18.01 Fall 2006 max min Figure 2: Search for max and min among critical points and endpoints Example 3. Find the open-topped can with the least surface area enclosing a fixed volume, V. h r Figure 3: Open-topped can. 1. Draw the picture. 2. Figure out what variables to use. (In this case, r, h, V and surface area, S.) 3. Figure out what the constraints are in the problem, and express them using a formula. In this example, the constraint is V = πr2 h = constant We’re also looking for the surface area. So we need the formula for that, too: S = πr2 + (2πr)h Now, in symbols, the problem is to minimize S with V constant. 2 Lecture 11 18.01 Fall 2006 4. Use the constraint equation to express everything in terms of r (and the constant V ). � � V V 2 h= ; S = πr + (2πr) 2πr πr2 5. Find the critical points (solve dS/dr = 0), as well as the endpoints. S will achieve its max and min at one of these places. dS 2V V = 2πr − 2 = 0 =⇒ πr3 − V = 0 =⇒ r3 = =⇒ r = dr π r � V π �1/3 We’re not done yet. We’ve still got to evaluate S at the endpoints: r = 0 and “r = ∞”. S = πr2 + 2V , r 0≤r<∞ 2 As r → 0, the second term, , goes to infinity, so S → ∞. As r → ∞, the first term πr2 goes r to infinity, so S → ∞. Since S = +∞ at each end, the minimum is achieved at the critical point r = (V /π)1/3 , not at either endpoint. s to ∞ to ∞ r Figure 4: Graph of S We’re still not done. We want to find the minimum value of the surface area, S, and the values of h. � �−2/3 � �1/3 � �1/3 V V V V V V r= ; h = 2 = � �2/3 = = π πr π π π π V π S = πr2 + 2 V =π r � V π �2/3 � + 2V V π �1/3 = 3π −1/3 V 2/3 Finally, another, often better, way of answering that question is to find the proportions of the h h (V /π)1/3 can. In other words, what is ? Answer: = = 1. r r (V /π)1/3 3 Lecture 11 18.01 Fall 2006 Example 4. Consider a wire of length 1, cut into two pieces. Bend each piece into a square. We want to figure out where to cut the wire in order to enclose as much area in the two squares as possible. 0 x 1 (1/4)x (1/4)(1-x) Figure 5: Illustration for Example 5. 2 The first square will have sides of length x4 . Its area will be x16 . The second square will have � �2 sides of length 1−4 x . Its area will be 1−4 x . The total area is then A = A� = �2 1−x 4 4 2x 2(1 − x) x 1 x 1 + (−1) = − + = 0 =⇒ 2x − 1 = 0 =⇒ x = 16 16 8 8 8 2 � x �2 � + So, one extreme value of the area is � 1 �2 A= 2 4 � 1 �2 + 2 = 4 1 32 We’re not done yet, though. We still need to check the endpoints! At x = 0, A = 02 + � 1−0 4 �2 = 1 16 At x = 1, � �2 1 1 A= + 02 = 4 16 4 Lecture 11 18.01 Fall 2006 By checking the endpoints in Fig. 6, we see that the minimum area was achieved at x = 12 . The maximum area is not achieved in 0 < x < 1, but it is achieved at x = 0 or 1. The maximum corresponds to using the whole length of wire for one square. Area 1/16 1/32 x 1/2 Figure 6: 1 Graph of the area function. Moral: Don’t forget endpoints. If you only look at critical points you may find the worst answer, rather than the best one. 5