Analysis of a Hyperbolic Paraboloid Surface with Curved Edges
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Transformation of Axes
19 Chapter 2 Transformation of Axes 2.1 Transformation of Coordinates It sometimes happens that the choice of axes at the beginning of the solution of a problem does not lead to the simplest form of the equation. By a proper transformation of axes an equation may be simplified. This may be accom- plished in two steps, one called translation of axes, the other rotation of axes. 2.1.1 Translation of axes Let ox and oy be the original axes and let o0x0 and o0y0 be the new axes, parallel respectively to the old ones. Also, let o0(h, k) referred to the origin of new axes. Let P be any point in the plane, and let its coordinates referred to the old axes be (x, y) and referred to the new axes be (x0y0) . To determine x and y in terms of x0, y0, h and x = h + x0 y = k + y0 20 Chapter 2. Transformation of Axes these equations represent the equations of translation and hence, the coeffi- cients of the first degree terms are vanish. 2.1.2 Rotation of axes Let ox and oy be the original axes and ox0 and oy0 the new axes. 0 is the origin for each set of axes. Let the angle x0 ox through which the axes have been rotated be represented by q. Let P be any point in the plane, and let its coordinates referred to the old axes be (x, y) and referred to the new axes be (x0, y0), To determine x and y in terms of x0, y0 and q : x = OM = ON − MN = x0 cos q − y0 sin q and y = MP = MM0 + M0P = NN0 + M0P = x0 sin q + y0 cos q 2.1. -
APPENDIX E Rotation and the General Second-Degree Equation
APPENDIX E Rotation and the General Second-Degree Equation Rotation of Axes • Invariants Under Rotation y y′ Rotation of Axes In Section 9.1, you learned that equations of conics with axes parallel to one of the coordinate axes can be written in the general form 2 1 2 1 1 1 5 x′ Ax Cy Dx Ey F 0. Horizontal or vertical axes θ Here you will study the equations of conics whose axes are rotated so that they are not x parallel to the x-axis or the y-axis. The general equation for such conics contains an xy-term. Ax2 1 Bxy 1 Cy2 1 Dx 1 Ey 1 F 5 0 Equation in xy-plane To eliminate this xy-term, you can use a procedure called rotation of axes. You want to rotate the x- and y-axes until they are parallel to the axes of the conic. (The rotated axes are denoted as the x9-axis and the y9-axis, as shown in Figure E.1.) After the After rotation of the x- and y-axes counter- rotation has been accomplished, the equation of the conic in the new x9y9-plane will clockwise through an angle u, the rotated have the form axes are denoted as the x9-axis and y9-axis. A9sx9d2 1 C9sy9d2 1 D9x9 1 E9y9 1 F9 5 0. Equation in x9y9-plane Figure E.1 Because this equation has no x9y9-term, you can obtain a standard form by complet- ing the square. The following theorem identifies how much to rotate the axes to eliminate an xy-term and also the equations for determining the new coefficients A9, C9, D9, E9, and F9. -
Rotation of Axes
ROTATION OF AXES ■■ For a discussion of conic sections, see In precalculus or calculus you may have studied conic sections with equations of the form Calculus, Early Transcendentals, Sixth Edition, Section 10.5. Ax2 ϩ Cy2 ϩ Dx ϩ Ey ϩ F 0 Here we show that the general second-degree equation 1 Ax2 ϩ Bxy ϩ Cy2 ϩ Dx ϩ Ey ϩ F 0 can be analyzed by rotating the axes so as to eliminate the term Bxy. In Figure 1 the x and y axes have been rotated about the origin through an acute angle to produce the X and Y axes. Thus, a given point P has coordinates ͑x, y͒ in the first coor- dinate system and ͑X, Y͒ in the new coordinate system. To see how X and Y are related to x and y we observe from Figure 2 that X r cos Y r sin x r cos͑ ϩ ͒ y r sin͑ ϩ ͒ y y Y P(x, y) Y P(X, Y) P X Y X r y ˙ ¨ ¨ 0 x 0 x X x FIGURE 1 FIGURE 2 The addition formula for the cosine function then gives x r cos͑ ϩ ͒ r͑cos cos Ϫ sin sin ͒ ͑r cos ͒ cos Ϫ ͑r sin ͒ sin ⌾ cos Ϫ Y sin A similar computation gives y in terms of X and Y and so we have the following formulas: 2 x X cos Ϫ Y sin y X sin ϩ Y cos By solving Equations 2 for X and Y we obtain 3 X x cos ϩ y sin Y Ϫx sin ϩ y cos EXAMPLE 1 If the axes are rotated through 60Њ, find the XY-coordinates of the point whose xy-coordinates are ͑2, 6͒. -
Rotation of Axes 1 Rotation of Axes
Rotation of Axes 1 Rotation of Axes At the beginning of Chapter 5 we stated that all equations of the form Ax2 + Bxy + Cy2 + Dx + Ey + F =0 represented a conic section, which might possibly be degenerate. We saw in Section 5.2 that the graph of the quadratic equation Ax2 + Cy2 + Dx + Ey + F =0 is a parabola when A =0orC = 0, that is, when AC = 0. In Section 5.3 we found that the graph is an ellipse if AC > 0, and in Section 5.4 we saw that the graph is a hyperbola when AC < 0. So we have classified the situation when B = 0. Degenerate situations can occur; for example, the quadratic equation x2 + y2 + 1 = 0 has no solutions, and the graph of x2 − y2 = 0 is not a hyperbola, but the pair of lines with equations y = x. But excepting this type of situation, we have categorized the graphs of all the quadratic equations with B =0. When B =6 0, a rotation can be performed to bring the conic into a form that will allow us to compare it with one that is in standard position. To set up the discussion, letusfirstsupposethatasetofxy-coordinate axes has been rotated about the origin by an angle θ, where 0 <θ<π=2, to form a new set ofx ^y^-axes, as shown in Figure 1(a). We would like to determine the coordinates for a point P in the plane relative to the two coordinate systems. y y yˆ yˆ P(x, y) P(xˆˆ, y) P(x, y) P(xˆˆ, y) r xˆ r xˆ B f B u u A x A x (a) (b) Figure 1 2 CHAPTER 5 Conic Sections, Polar Coordinates, and Parametric Equations First introduce a new pair of variables r and φ to represent, respectively, the distance from P to the origin and the angle formed by thex ^-axis and the line connecting the origin to P , as shown in Figure 1(b). -
Rotation of Axes
ROTATION OF AXES ■■ For a discussion of conic sections, see In precalculus or calculus you may have studied conic sections with equations of the form Review of Conic Sections. Ax2 ϩ Cy2 ϩ Dx ϩ Ey ϩ F 0 Here we show that the general second-degree equation 1 Ax2 ϩ Bxy ϩ Cy2 ϩ Dx ϩ Ey ϩ F 0 can be analyzed by rotating the axes so as to eliminate the term Bxy. In Figure 1 the x and y axes have been rotated about the origin through an acute angle to produce the X and Y axes. Thus, a given point P has coordinates ͑x, y͒ in the first coor- dinate system and ͑X, Y͒ in the new coordinate system. To see how X and Y are related to x and y we observe from Figure 2 that X r cos Y r sin x r cos͑ ϩ ͒ y r sin͑ ϩ ͒ y y Y P(x, y) Y P(X, Y) P X Y X r y ˙ ¨ ¨ 0 x 0 x X x FIGURE 1 FIGURE 2 The addition formula for the cosine function then gives x r cos͑ ϩ ͒ r͑cos cos Ϫ sin sin ͒ ͑r cos ͒ cos Ϫ ͑r sin ͒ sin ⌾ cos Ϫ Y sin A similar computation gives y in terms of X and Y and so we have the following formulas: 2 x X cos Ϫ Y sin y X sin ϩ Y cos By solving Equations 2 for X and Y we obtain 3 X x cos ϩ y sin Y Ϫx sin ϩ y cos EXAMPLE 1 If the axes are rotated through 60Њ, find the XY-coordinates of the point whose xy-coordinates are ͑2, 6͒. -