# Problem 1 (10 pts) Find the radius of convergence and interval of convergence of the series

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1 1 Problem 1 (10 pts) Find the radius of convergence and interval of convergence of the series a n n=1 n(x + 2) n 5 n 1. n(x + 2)n Solution: Do the ratio test for the absolute convergence. Let a n =. Then, 5 n 1 a n+1 = (n + 1)(x + 2) n+1 5 n 1 5 n n(x + 2) n = n + 1 x + 2 5n n x x + 2 Hence, the power series converges absolutely if < 1. From this, we can see that the 5 radius of convergence is R = 5. Also, the interval of convergence is 5 < x + 2 < 5, i.e., 7 < x < 3. Let s check the convergence when x is at the boundary points. For x = 7, the series becomes: n( 5) n 5 n 1 = 5n( 1) n. n=1 Since lim 5n( 1) n 0, this series does not converge (the nth Term Test for Divergence). n So, we cannot include x = 7 in the interval of convergence. How about x = 3? This leads to n5 n 5 n 1 = 5n. n=1 Clearly this diverges (again via the nth Term Test for Divergence). So, we cannot include x = 3 in the interval of convergence either. Hence the interval of convergence is: ( 7,3) or 7 < x < 3. n=1 n=1

2 2 Problem 2 (10 pts) Consider the function f (x) = ln x. (a) (5 pts) Approximate f (x) by a Taylor polynomial of degree 2 centered at x = 2. (b) (5 pts) How accurate is this approximation when 1 x 3? Solution to (a): Since f (x) = x 1, f (x) = x 2, f (x) = 2x 3, by Taylor s formula, we have f (x) = ln x = P 2 (x) + R 2 (x) = f (2) + f (2)(x 2) + f (2) 2 (x 2)2 + f (c) (x 2) 3 2 < c < x or x < c < 2 3! = ln (x 2) 1 8 (x 2) c 3 (x 2)3. Hence, we have P 2 (x) = ln (x 2) 1 8 (x 2)2. Solution to (b): The approximation error is: R 2 (x) = 1 3c 3 x c 3 < 1 3, since x 2 1 and 1 < c < 3. Therefore, the accuracy of approximation by P 2 (x) for 1 x 3 is bounded by , which is the worst-case scenario. 3

3 3 Problem 3 (10 pts) State and prove the Cauchy-Schwarz Inequality. Note that you also need to state and prove the condition for the equality to hold. Solution: Let u, v be any two vectors in R n, n N. Then, the Cauchy-Schwarz Inequality is u v u v where the equality holds if and only if u and v are parallel to each other. The proof of this inequality is based on the definition of the dot product. In other words, let θ be the angle between u and v. Then, u v = u v cosθ = u v cosθ u v since clearly cos θ 1. From the above derivation, it is also clear that the equality holds if and only if cosθ = 0 (which includes the case where either u or v is the zero vector), which is the same as saying that u and v are parallel to each other.

4 4 Problem 4 (10 pts) Let P(1, 4, 6), Q( 2, 5, 1), R(1, 1, 1). (a) (5 pts) Find the area of the triangle PQR. Hint: Length of the cross product of two vectors is equal to the area of a parallelogram formed by those two vectors. (b) (5 pts) Find the distance from P to the line QR. Solution to (a): We have PQ = 2 1,5 4, 1 6 = 3,1, 7, and PR = 1 1, 1 4,1 6 = 0, 5, 5. So, PQ PR = Hence, the area of PQR is i j k = 40i 15j + 15k = 40, 15, PQ PR = , 15,15 = = = Solution to (b): You can compute the distance d from P to the line QR by the formula based on the cross product, i.e., QP QR d =. QR Now, notice that QP QR = PQ PR since both amounts to the area of the same parallelogram, which is the twice as the area of the triangle PQR. Since QR = 3, 6,2, we have d = ( 6) = Alternatively, you can reason as follows: the distance d can be computed by the following basic formula Area of PQR = 1 2 d. QR Since QR = 3, 6,2, we have d = ( 6) =

5 5 Problem 5 (10 pts) Find the limit if it exists, or show that the limit does not exist. (a) (5 pts) (b) (5 pts) lim (x,y) (0,0) y 4 x 4 + 3y 4. Hint: Consider (x, y) (0,0) along the line y = mx. lim (x,y) (0,0) x y x 2 + y. 2 Hint: Consider the limit in the polar coordinates (r,θ). Solution to (a): Consider (x, y) (0, 0) along the line y = mx for some m R. Then, lim (x,y) (0,0) y 4 x 4 + 3y 4 = lim x 0 m 4 x 4 x 4 + 3m 4 x 4 = lim x 0 m m 4 = m m 4. This value depends on m. Since the limit depends on how the point (x, y) approaches (0,0), the limit does not exist. Solution to (b): Consider the polar coordinates: x = r cosθ, y = r sinθ, where r 0, 0 θ < 2π. lim (x,y) (0,0) x y x 2 + y = lim 2 r 0 r 2 cosθ sinθ r 2 (cos 2 θ + sin 2 θ) r 2 cosθ sinθ = lim = lim r cosθ sinθ = 0. r 0 r r 0 This is true regardless of the direction of approach θ. Hence, the limit exists and its value is 0.

6 6 Problem 6 (10 pts) Verify that the function of two variables u(x, t) = e α2 k 2t sinkx, where k is an arbitrary positive integer is a solution of the heat conduction equation over the interval x [0,π] u t = α 2 u xx, with the boundary condition Solution: This is a quite straightforward problem. Hence, u x = u x = e α2 k 2t k coskx, u(0, t) = u(π, t) = 0 for all t 0. u t = u t = α2 k 2 e α2 k 2t sinkx. u xx = 2 u x 2 = e α2 k 2t ( k 2 )sinkx. α 2 u xx = α 2 k 2 e α2 k 2t sinkx = u t. Finally, we can easily check that the boundary condition is satisfied since for any t 0. u(0, t) = e α2 k 2t sink0 = 0, u(π, t) = e α2 k 2t sinkπ = 0,

7 7 Problem 7 (10 pts) Assuming that the equation sin x + cos y = sin x cos y. defines y as a differentiable function of x, use the Implicit Differentiation Theorem to find dy dx. Solution: Let F (x, y) = sin x + cos y sin x cos y = 0. Then, the Implicit Differentiation Theorem says that dy dx = F x. F y Now, F x (x, y) = cos x cos x cos y, F y (x, y) = sin y + sin x sin y. Hence, dy dx cos x cos x cos y cos x (1 cos y) = = sin y + sin x sin y sin y (1 sin x), as long as F y (x, y) = sin y (sin x 1) 0.

8 8 Problem 8 (10 pts) Find the direction in which f (x, y, z) = ln(x y 2 z 3 ) (a) (3 pts) Increases most rapidly at the point (1,2,3), (b) (3 pts) Decreases most rapidly at the point (1,2,3). (c) (4 pts) Does not change (i.e., is flat) at the point (1,2,3). Solution to (a): First of all, note that f (x, y, z) = ln(x y 2 z 3 ) = ln x + 2ln y + 3ln z. Now, the gradient vector is 1 f (x, y, z) = f x, f y, f z = x, 2 y, 3. z So, f (1, 2, 3) = 1, 1, 1. Now, f increases most rapidly in the direction of f. Hence, this direction is 1,1,1. (It is up to you whether you want to make it to the normal vector of length 1, i.e., 1 3, 1 1 3,, but it is not necessary in this case.) 3 Solution to (b): f decreases most rapidly in the direction of f. Hence, it is 1,1,1 = 1, 1, 1. Solution to (c): f does not change in the direction orthogonal to f. Let u = u 1,u 2,u 3 be this directional vector. Then, u f (1,2,3) = 0 u = u 1,u 2,u 3 1,1,1 = 0 u 1 + u 2 + u 3 = 0. So, along any direction u = u 1,u 2,u 3 satisfying u 1 + u 2 + u 3 = 0, this function f does not change (or is flat) at the point (1,2,3).

9 9 Problem 9 (10 pts) (a) (5 pts) Show the equation of the tangent plane at the point P 0 (x 0, y 0, z 0 ) on the ellipsoid x 2 a 2 + y2 b 2 + z2 c 2 = 1 is x 0 x a 2 + y 0 y b 2 + z 0 z c 2 = 1. (b) (5 pts) Compute the normal line to the same ellipsoid at the same point P 0. Furthermore, compute the point where this normal line intersects with the x y-plane. Assume z 0 0. Solution to (a): Let F (x, y, z) = x2 a 2 + y2 b 2 + z2 1 = 0. The equation of the tangent plane to c2 F at (x 0, y 0, z 0 ) is Since we have F x (x 0, y 0, z 0 )(x x 0 ) + F y (x 0, y 0, z 0 )(y y 0 ) + F z (x 0, y 0, z 0 )(z z 0 ) = 0. F x (x, y, z) = 2x a 2, the equation of the tangent plane becomes F x(x, y, z) = 2y b 2, F x(x, y, z) = 2z c 2, 2x 0 a 2 (x x 0) + 2y 0 b 2 (y y 0) + 2z 0 c 2 (z z 0) = 0 x 0(x x 0 ) a 2 + y 0(y y 0 ) b 2 + z 0(z z 0 ) c 2 = 0 x 0x a 2 + y 0y b 2 + z 0z c 2 = x2 0 a 2 + y2 0 b 2 + z2 0 c 2 = 1. In the last line, the righthand side becomes 1 since the point P 0 (x 0, y 0, z 0 ) is on this ellipsoid. Solution to (b): At the point P 0 (x 0, y 0, z 0 ), the gradient vector is F (x 0, y 0, z 0 ) = 2x 0 a 2, 2y 0 b 2, 2z 0 c. 2 The normal line passes through the point P 0 is parallel to this gradient vector. Hence, the equation of this normal line is ( ) x = x 0 + t 2x 0 = x a t ( a 2 ) y = y 0 + t 2y 0 = y b t, ( b 2 ) z = z 0 + t 2z 0 = z c t c 2 where t is an arbitrary real value. As for the intersection of this normal line and the x y-plane, because x y-plane is the same as the equation z = 0, we seek the value of t that makes z = 0. This means that t = c2 2 since z 0 0. Hence, we insert this t into the above equation to get the ) ) coordinate of the intersection (x, y, z) = (x 0 (1 ), c2 y 0 (1 c2, 0. a 2 b 2

10 10 Problem 10 (10 pts) Find the linearization of f (x, y, z) = x 2 + y 2 + z 2 at the point (3,2,6). Then use it to approximate f (3.1,1.9,6.2) by the three decimal point number. Note that the true value of f (3.1,1.9,6.2) is in the three decimal point number. Solution: The linearization of f (x, y, z) at the point (x 0, y 0, z 0 ) is L(x, y, z) = f (x 0, y 0, z 0 ) + f x (x 0, y 0, z 0 )(x x 0 ) + f y (x 0, y 0, z 0 )(y y 0 ) + f z (x 0, y 0, z 0 )(z z 0 ). We have f x (x, y, z) = 1 2 (x2 + y 2 + z 2 ) 1 2 2x = x(x 2 + y 2 + z 2 ) 1 2. Since f (x, y, z) is symmetric with respect to x, y, z, we immediately have f y (x, y, z) = y(x 2 + y 2 + z 2 ) 1 2, f z (x, y, z) = z(x 2 + y 2 + z 2 ) 1 2. Now, at the point (3,2,6), we have f (3,2,6) = = 49 = 7. Hence, L(x, y, z) = (x 3) (y 2) + 6 (z 6). 7 Let s now use this to approximate f (3.1,1.9,6.2). L(3.1,1.9,6.2) = (3.1 3) (1.9 2) (6.2 6) = On the other hand, the true value is f (3.1,1.9,6.2) = as the hint suggests. So, the approximation is quite close to the true value and the error is f (3.1, 1.9, 6.2) L(3.1, 1.9, 6.2) = Observe that this linear approximation is quite good!

11 11 Problem 11 (10 pts) Find the absolute maximum and minimum values of f (x, y) = x 2 x y + y 2 + 1, on the closed triangular domain bounded by the lines x = 0, y = 2, y = 2x, i.e., Ω = {(x, y) x 0, y 2, y 2x}. Solution: The domain Ω is the shaded triangle in the figure. Figure 1: A triangular domain Ω. Step 1: Compute all the critical points in Ω. f x = 2x y = 0, f y = x + 2y = 0 y = 2x, x = 2y x = 4x x = 0. Hence, x = y = 0 is the only critical point in Ω. Now we need to check whether this point is local min., local max., or a saddle point. To do so, we compute f xx (x, y) = 2, f y y (x, y) = 2, f x y (x, y) = 1. From these, we do the Second Derivative Test: D(0,0) = f xx (0,0)f y y (0,0) ( f x y (0,0) ) 2 = 2 2 ( 1) 2 = 3 > 0. Since D(0,0) > 0 and f xx (0,0) = 2 > 0, the value f (0,0) = 1 is the local minimum.

12 12 Step 2 : Find the extreme values at the boundary of Ω. On x = 0: f (0, y) = y 2 + 1, 0 y 2. So, on this boundary, the minimum is f (0,0) = 1 while the maximum is f (0, 2) = 5. On y = 2: f (x,2) = x 2 2x + 5 = (x 1) 2 + 4, 0 x 1. So, on this boundary, the minimum is f (1,2) = 4 while the maximum is f (0,2) = 5. On y = 2x: f (x,2x) = 3x 2 + 1, 0 x 1. So, on this boundary, the minimum is f (0,0) = 1 while the maximum is f (1,2) = 4. Step 3: Find the largest and the smallest values from the results of Steps 1 and 2. Combining all the results we got so far, we can easily conclude that the absolute maximum is f (0,2) = 5 while the absolute minimum is f (0,0) = 1.

13 13 Problem 12 (10 pts) Use Lagrange multipliers to find the maximum and minimum values of the function f (x, y) = e x y subject to x 2 + y 2 = 1. Note that we only consider the real values for x and y, not the complex values. Solution: Let g (x, y) = x 2 + y 2 1 = 0 be the constraint. Let s find the points (x, y) and the parameter λ satisfying f = λ g, g (x, y) = 0. (1) We have Hence, Eq. (1) leads to f = f x, f y = y e x y, x e x y, g = g x, g y = 2x,2y. y e x y = 2λx (2) x e x y = 2λy (3) x 2 + y 2 = 1. (4) We can see that λ 0 because if λ = 0, then x = y = 0 from Eqn s (2) and (3), which contradicts Eq. (4). Similarly, x 0 as well as y 0. Hence, we can divide Eq. (2) by Eq. (3) to get y e x y x e x y = 2λx 2λy y x = x y x2 = y 2. This implies x = y or x = y. Insert these into Eq. (4) gives us 2x 2 = 1 or x = ± 1. So, ( ) ( 2 1 we have (x, y) = 2, 1 1 2, 2, 1 ) (, 1 1, ), or ( 2 1, 1 ) 2. From Eq. (2), the e corresponding λ s can be easily calculated as λ = 2, 1 2 e, 1 e 2 e, or 2, respectively. Hence, ( ) 1 f 2, 1 2 = f ( 2 1, 2 1 ) = e, ( 1 f 2, 1 ) ( = f 1 ) 1, = e Therefore, the maxima are ( ) 1 f 2, 1 2 = f ( 2 1, 2 1 ) = e, and the minima are ( 1 f 2, 1 ) ( = f 1, 2 2 ) 1 = 1. 2 e

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