#$&* course 272 12/11 5pm If your solution to a stated problem does not match the given solution, you should self-critique per instructions at
.............................................
Given Solution: `a fx = 2x + 6y ; fy = 6x + 20 y - 4, where fx and fy mean x and y partial derivatives of f. fxx is the x derivative of fx and is therefore 2 fyy is the y derivative of fy and is therefore 20 fxy is the y derivative of fx and is therefore 6. Note that fyx is the x derivative of fy and is therefore 6. fxy and fyx are always the same, provided the derivatives exist. fx = 0 and fy = 0 if 2x + 6y = 0 and 6x + 20y - 4 = 0. This is a system of two equations in two unknowns. You can solve the system by any of several methods. Multiplying the first equation by -3 and adding the resulting equations, for example, gives you 2 y = 4 so that y = 2. Substituting y = 2 into the first equation gives you 2x + 6 * 2 = 0, which has solution x = 6. Whatever method you use, the solution of this system is x = -6, y = 2. To determine the nature of the critical point at (-6, 2) you have to look at fxx, fyy and fxy. We have fxx = 2 fyy = 20 fxy = 6. This is a system of two equations in two unknowns. You can solve the system by any of several methods. Multiplying the first equation by -3 and adding the resulting equations, for example, gives you 2 y = 4 so that y = 2. Substituting y = 2 into the first equation gives you 2x + 6 * 2 = 0, which has solution x = -6. Whatever method you use, the solution of this system is x = -6, y = 2. To determine the nature of the critical point at (-6, 2) you have to evaluate the quantity fxx * fyy - 4 fxy^2. We have fxx = 2 fyy = 20 fxy = 6. So fxx * fyy - fxy^2 = 2 * 20 - 6^2 = 4. This quantity is positive, so you have either a maximum or a minimum. Since fxx and fyy are both positive the graph is concave upward in all directions and the point is a minimum. The coordinates of the point are (-6, 2, (-6)^2+6 * (-6) * 2 + 10* 2^2 - 4 * 2 + 4 ) = (-6, 2, 0) &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique Rating: 3 ********************************************* Question: `qQuery problem 7.5.28 extrema of x^3+y^3 -3x^2+6y^2+3x+12y+7 List the relative extrema and the saddle points of the function and tell which is which, and how you obtained each. YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY Your solution: x^3+y^3 -3x^2+6y^2+3x+12y+7 f(x) = 3x^2 - 6x + 3 = 3(x^2 - 2x + 1) = 3 (x-1)^2 f(y) = 3y^2 + 12y + 12 = 3(y^2 + 4x + 4) = 3 (x+2)^2 f(x) = 0 so 3 (x-1)^2= 0 x= 1 f(y)= 0 so 3 (x+2)^2 = 0 y=-2 f(xx)=6x-6 f(yy)6y+12 f(xy)=f(yx)=0 critical point (1, -2, 0) (fxx)*(fyy)-(fxy) = 0*0-0 = 0 which is inconclusive. confidence rating #$&*: 3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.............................................
Given Solution: `a We have fx = 3 x^2 + 6 x + 3 and fy = 3 y^2 + 12 y + 12. Factoring we get fx = 3 ( x^2 - 2x + 1) = 3 ( x-1)^2 and fy = 3 ( y^2 + 4y + 4) = 3 ( y+2)^2. So fx = 0 when 3(x-1)^2 = 0, or x = 1 and fy = 0 when 3(y+2)^2 = 0 or y = -2. We get fxx = 6 x - 6 (the x derivative of fx) fyy = 6 y + 12 (the y derivative of fy) and fxy = 0 (the y derivative of fx, which is also equal to fyx, the x derivative of fy) At the critical point x = 1, y = -2 we get fxx = fyy = 0. So the test for max, min and critical point gives us fxx*fyy - fxy^2 = 0, which is inconclusive--it tells us nothing about max, min or saddle point. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Self-critique (if necessary): ok ------------------------------------------------ Self-critique Rating: 3 " Self-critique (if necessary): ------------------------------------------------ Self-critique rating: " Self-critique (if necessary): ------------------------------------------------ Self-critique rating: #*&!