Mathematics 309 Conic sections and their applicationsn. Chapter 2. Quadric figures. ai,j x i x j + b i x i + c =0. 1. Coordinate changes

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1 Mathematics 309 Conic sections and their applicationsn Chapter 2. Quadric figures In this chapter want to outline quickl how to decide what figure associated in 2D and 3D to quadratic equations look like. In an number of dimensions, a quadratic equation is one of the form ai,j i j + b i i + c 0 or in other words one where all terms have degree at most two. In addition we shall ignore the degenerate case where there are no terms of degree two. In the first Chapter, I have defined the conic sections in terms of simple quadratic equations, and in dealing with the characterization in terms of focus and directri we have looked at translations of these equations. The ke to our discussion in this Chapter will be making coordinate changes so the equation has a particularl simple form. As an application, we will also discuss conic sections, the curves we get b intersecting circular cones with planes. One application will be to see wh it is that when ou look at a circle in space in an position at all, what ou reall see is an ellipse. This is not at all an evident fact, as simple sketches should demonstrate. For the sake of simplicit, I shall often assume we are working in 2D. Onl the algebra becomes more complicated in higher dimensions the basic concepts remain the same. 1. Coordinate changes One of the main basic points is that quadratic equations become simple if we make a suitable change of coordinates. Since we don t want to change the shape of things we are looking at, we shall usuall restrict ourselves to orthogonal changes of coordinates. This means that if (, ) are the original coordinates and (, ) the new coordinates for the same point, the distances between two points are given b (2 1 ) 2 +( 2 1 ) 2 (,2,1 ) 2 +(,2,1 ) 2. Even with this restriction we can alwas simplif quadratic equations to a number of simple tpes, so that in the end we shall see that quadratic equations, with a few simple eceptions, define ellipses, parabolas, or hperbolas. I have essentiall defined these to be the curves defined in an orthogonal coordinate sstem b equations ( ) 2 ( ) a b ( ) 2 ( ) + 0 a b ( ) 2 ( ) 2 1 a b Let me start with an eample. Consider the curve You can guess b plotting a few points that this curve looks like this:

2 Quadric figures 2 In other words, it looks like an ellipse whose major ais is along the line and whose minor ais is along the line. Some calculation will tell ou that the size of the semi-major ais is 2 and that of the semi-minor ais is 2/3. We can therefore sa that this is the ellipse if we choose our coordinate sstem to match the aes ±. This is the sort of thing we want to generalize. The first question to be answered is what happens to quadratic equations when we make a coordinate change? We will allow at first the most general kind of coordinate change. Such a change is effected b an affine transform, and can be epressed as a combination of a linear change of coordinates A where A is an invertible 2 2 matri, with a translation + τ τ. In the eample above, we make a linear change where A is the rotation matri cos θ sin θ, θ 45. sin θ cos θ Wh is this? Because the transformation of coordinate frames from one coordinate sstem to the other is b this rotation. So we analze how quadratic equations change in two phases. In both phases the main trick is to write a homogeneous quadratic epression in terms of matri multiplication: a 2 + b + c 2 a b/2 b/2 c Q as ou can calculate ourself. Here the important thing is that Q is a smmetric matri. Similarl we can also write the linear epression in a quadratic function in terms of matri multiplication: a + b a b l.

3 Quadric figures 3 where l is a row vector. If we change coordinates we change Q to Q t AQA and we change l to l la. Suppose we translate coordinates Then the homogeneous function corresponding to Q becomes the epression with the same quadratic term Q, but linear term τ τ and constant term + τ τ Q Eercise 1.1. Under translation, what happens to a linear function corresponding to the row vector l? τ τ τ τ. 2. The basic classification Since Q is a smmetric matri we can diagonalize it. Recall from our course on linear algebra that it has real eigenvalues (as opposed to ones which are possibl comple) and that we can choose its eigenvectors to be perpendicular to each other. Furthermore we can normalize the eigenvectors to have length 1. In other words we can find an orthogonal matri A such that A 1 λ 0 QA 0 µ where λ and µ are the eigenvalues of Q and the columns of A are its eigenvectors. Recall that an orthogonal matri A is one such that t A A 1. Equivalentl, its columns are all perpendicular to each other and have length 1. In other words we can make a coordinate change so the quadratic epression becomes The matri associated to λ 2 + µ is 1 1/2 1/2 1 with eigenvalues 1/2, 3/2 and eigenvectors 1/ 2 ±1/. 2 As a result of the first phase, we have changed our quadratic epression into one of the form a 2 + b 2 + c + d + e. What we do net depends on what the epression we get. We are assuming that we are not dealing with a linear epression, in which case it cannot be that both a and b vanish. We can swap and if necessar to make b 0. If we set + t, we get an epression a 2 + b( + t) 2 + c + d( + t)+e a 2 + b 2 +2bt + bt 2 + c + d + e a 2 + b 2 + c +(2bt + d) + bt 2 + dt + e

4 Quadric figures 4 and if we choose 2bt + d 0, t d/2b (possible since b 0) we can make the linear term in vanish. If a 0we now have an epression of the form c + b 2 + d. And if a 0we can similarl translate as well to obtain an epression of the form We can summarize: If a 2 + b 2 + d. f(, ) a 2 + b + c 2 + d + e + f is an quadratic epression, then we combine an orthogonal change of coordinates with a translation of coordinates to make the epression into one of the two forms a 2 + b + c a 2 + b 2 + c. As a corollar, we see that an curve of the form f(, ) 0 is equivalent to one of the form or a 2 + b + c 0 a 2 + b 2 + c 0. Eercise 2.1. Among the curves we get will be parabolas, ellipses, and hperbolas. What else? 3. Circles in perspective Suppose we place a circle somewhere in space, placing the etra condition on it that the plane containing it does not pass through the origin. Collect together all lines passing through pairs O, P where O is the origin and P is a point on the circle. These lines will make up a kind of oblique cone whose verte is at the origin. For eample, if the circle we start with is the circle R 2 of radius R on the plane z 1we get the right circular cone z 2 ( )/R 2. This cone has the propert that if we slice it with an horizontal plane z c we get a circle (whose radius grows proportionall to c). The problem we want to discuss here is: if in the general case we slice the cone b the plane z 1, what geometrical figure do we get?

5 Quadric figures 5 This corresponds ver closel to the pschological problem of determining what a circle located somewhere in space looks like, since the image of the circle is the figure traced out b all the light ras from the circle passing through the lens of the ee (the origin) hitting the back of the ee (roughl the plane z h for some small h). This optical image is essentiall the figure on the plane z 1, scaled down b h, and inverted. There are several steps in figuring out the solution. The point of the first few step is to obtain the equation of the cone. We then set z 1in this equation to get the equation of the slice, and appl the techniques described above (eigenvalues, etc.) to see what this curve looks like. We find the equation of the cone b appling an etremel fundamental principle which can be applied in far more general circumstances. The principle is to choose a coordinate sstem in which the equation of the cone is simple, and then to figure out how these coordinates relate to the ones we start with. This principle has been implicitl encountered alread, where we chose a coordinate sstem related to eigenvectors to see the real shape of a quadric curve. But the new feature here, one that is in fact quite natural in spite of being new, is that the coordinate sstem we choose is oblique, that is to sa not at all orthogonal. An analogous eample in 2D should clarif things. Consider the 2D cone which looks like this:

6 Quadric figures 6 We think of it as being constructed in the following wa: we choose the two points ( 1, 1) and (1, 1) and make our cone b drawing lines to the origin through them. Of course the lines we draw are ± and the equation of the cone made up of these two lines is the quadratic equation Now suppose we choose two different points, sa (1, 1) and (3, 1). What is the equation of this cone? This is such a simple problem that there are several answers, but the one that turns out to be most useful is to choose a coordinate sstem in which these points have the same values as in the previous problem. That coordinate sstem is indicated in this picture: Let (, ) be the coordinates in this picture. The frame for this coordinate sstem is made up of the vectors f 1 (1, 0) and f 2 (2, 1). The relationship between the two coordinate sstems is given, according to what we have seen before, b the formula 1 2 F

7 Quadric figures 7 where the columns of the matri F are the vectors f 1 and f 2. Now the quadratic epression 2 2 can be written in terms of matri multiplication as which is the same as since Here t F F F 1, t F 1. F 1 2, F so the epression in (, ) is which corresponds to the quadratic epression So the equation of our cone in the (, ) coordinate sstem is You can check this b seeing if the proper points are on it This is a lot of work to write down the equation for a pair of lines through the origin, but the principles remain the same in an number of dimensions. Here is what we do for a cone through a circle of radius R in space: (1) Choose a coordinate sstem in which the equation of the cone is z The frame for this coordinate sstem will be made up of three vectors f 1, f 2, f 3 with these properties: (a) The vector f 3 has its head at the centre of the circle. (b) The two vectors f 1 and f 2 have length R, are perpendicular to each other, and lie in the plane through the origin parallel to the plane in which the circle lies. (2) If F f 1 f 2 f 3 Then the smmetric matri corresponding to the quadratic epression we are looking for is t F F