Analytic Geometry

REVIEW OF ANALYTIC GEOMETRY The points in a plane can be identified with ordered pairs of real numbers. We start by drawing two perpendicular coordinate lines that intersect at the origin O on each line. Usually one line is horizontal with positive direction to the right and is called the x-axis; the other line is vertical with positive direction upward and is called the y-axis. Any point P in the plane can be located by a unique ordered pair of numbers as follows. Draw lines through P perpendicular to the x- and y-axes. These lines intersect the axes in points with coordinates a and b as shown in Figure 1. Then the point P is assigned the ordered pair ͑a, b͒. The first number a is called the x-coordinate of P; the second number b is called the y-coordinate of P. We say that P is the point with coordinates ͑a, b͒, and we denote the point by the symbol P͑a, b͒. Several points are labeled with their coordinates in Figure 2. y y 4 P(a, b) 4 b 3 3 (1, 3) (_2, 2) II 2 I 2 1 1 (5, 0) _3 _2 _1 O 1 2345x _3 _2 _1 0 1 2345x _1 _1 a _2 _2 III IV (_3, _2)) _3 _3 _4 _4 (2, _4) FIGURE 1 FIGURE 2 By reversing the preceding process we can start with an ordered pair ͑a, b͒ and arrive at the corresponding point P. Often we identify the point P with the ordered pair ͑a, b͒ and refer to “the point ͑a, b͒.” [Although the notation used for an open interval ͑a, b͒ is the same as the notation used for a point ͑a, b͒, you will be able to tell from the context which meaning is intended.] This coordinate system is called the rectangular coordinate system or the Cartesian coordinate system in honor of the French mathematician René Descartes (1596–1650), even though another Frenchman, Pierre Fermat (1601–1665), invented the principles of analytic geometry at about the same time as Descartes. The plane supplied with this coordinate system is called the coordinate plane or the Cartesian plane and is denoted byޒ 2 . The x- and y-axes are called the coordinate axes and divide the Cartesian plane into four quadrants, which are labeled I, II, III, and IV in Figure 1. Notice that the first quad- rant consists of those points whose x- and y-coordinates are both positive. EXAMPLE 1 Describe and sketch the regions given by the following sets. (a)͕͑x, y͒ Խ x ജ 0͖ (b)͕͑x, y͒ Խ y ෇ 1͖ (c ) {͑x, y͒ Խ Խ y Խ Ͻ 1} SOLUTION (a) The points whose x-coordinates are 0 or positive lie on the y-axis or to the right of it as indicated by the shaded region in Figure 3(a). y y y y=1 y=1 0 x 0 x 0 x y=_1 FIGURE 3 (a) x у 0 (b) y=1 (c) |y|<1 1 Thomson Brooks-Cole copyright 2007 2 ■ REVIEW OF ANALYTIC GEOMETRY (b) The set of all points with y-coordinate 1 is a horizontal line one unit above the x-axis [see Figure 3(b)]. (c) Recall from Review of Algebra that Խ y Խ Ͻ 1 if and only if Ϫ1 Ͻ y Ͻ 1 The given region consists of those points in the plane whose y-coordinates lie between Ϫ1 and 1. Thus, the region consists of all points that lie between (but not on) the horizontal lines y ෇ 1 and y ෇ Ϫ1. [These lines are shown as dashed lines in Figure 3(c) to indicate that the points on these lines don’t lie in the set.] y Recall from Review of Algebra that the distance between points a and b on a number line P™(¤, fi) fi is Խ a Ϫ b Խ ෇ Խ b Ϫ a Խ. Thus, the distance between points P1͑x1, y1͒ and P3͑x2, y1͒ on a horizontal line must be Խ x2 Ϫ x1 Խ and the distance between P2͑x2, y2 ͒ and P3͑x2, y1͒ on |fi-›| Ϫ P¡(⁄, ›) a vertical line must be Խ y2 y1 Խ. (See Figure 4.) › To find the distance Խ P1P2 Խ between any two points P1͑x1, y1͒ and P2͑x2, y2 ͒, we note |¤-⁄| P£(¤, ›) that triangle P1P2 P3 in Figure 4 is a right triangle, and so by the Pythagorean 0 ⁄¤x Theorem we have ෇ s 2 ϩ 2 ෇ s Ϫ 2 ϩ Ϫ 2 FIGURE 4 Խ P1P2 Խ Խ P1P3 Խ Խ P2P3 Խ Խ x2 x1 Խ Խ y2 y1 Խ 2 2 ෇ s͑x2 Ϫ x1͒ ϩ ͑y2 Ϫ y1͒ Distance Formula The distance between the points P1͑x1, y1͒ and P2͑x2, y2 ͒ is 2 2 Խ P1P2 Խ ෇ s͑x2 Ϫ x1͒ ϩ ͑y2 Ϫ y1͒ For instance, the distance between ͑1, Ϫ2͒ and ͑5, 3͒ is s͑5 Ϫ 1͒ 2 ϩ ͓3 Ϫ ͑Ϫ2͔͒ 2 ෇ s4 2 ϩ 5 2 ෇ s41 CIRCLES y An equation of a curve is an equation satisfied by the coordinates of the points on the P(x, y) curve and by no other points. Let’s use the distance formula to find the equation of a cir- r cle with radius r and center ͑h, k͒. By definition, the circle is the set of all points P͑x, y͒ whose distance from the center C͑h, k͒ is r. (See Figure 5.) Thus,P is on the circle if and C(h, k) only if Խ PC Խ ෇ r. From the distance formula, we have s͑x Ϫ h͒2 ϩ ͑ y Ϫ k͒2 ෇ r or equivalently, squaring both sides, we get 0 x ͑x Ϫ h͒2 ϩ (y Ϫ k͒2 ෇ r 2 FIGURE 5 This is the desired equation. Equation of a Circle An equation of the circle with center ͑h, k͒ and radius r is ͑x Ϫ h͒2 ϩ (y Ϫ k͒2 ෇ r 2 In particular, if the center is the origin ͑0, 0͒, the equation is x 2 ϩ y 2 ෇ r 2 For instance, an equation of the circle with radius 3 and center ͑2, Ϫ5͒ is ͑x Ϫ 2͒2 ϩ (y ϩ 5͒2 ෇ 9 Thomson Brooks-Cole copyright 2007 REVIEW OF ANALYTIC GEOMETRY ■ 3 EXAMPLE 2 Sketch the graph of the equation x 2 ϩ y 2 ϩ 2x Ϫ 6y ϩ 7 ෇ 0 by first show- ing that it represents a circle and then finding its center and radius. SOLUTION We first group the x-terms and y-terms as follows: y ͑x 2 ϩ 2x͒ ϩ (y 2 Ϫ 6y͒ ෇ Ϫ7 Then we complete the square within each grouping, adding the appropriate constants (_1, 3) (the squares of half the coefficients of x and y) to both sides of the equation: ͑x 2 ϩ 2x ϩ 1͒ ϩ (y 2 Ϫ 6y ϩ 9͒ ෇ Ϫ7 ϩ 1 ϩ 9 or ͑x ϩ 1͒2 ϩ (y Ϫ 3͒2 ෇ 3 0 1 x Comparing this equation with the standard equation of a circle, we see that h ෇ Ϫ1, FIGURE 6 k ෇ 3, and r ෇ s3, so the given equation represents a circle with center ͑Ϫ1, 3͒ and ≈+¥+2x-6y+7=0 radius s3. It is sketched in Figure 6. LINES To find the equation of a line L we use its slope, which is a measure of the steepness of the line. Definition The slope of a nonvertical line that passes through the points P1͑x1, y1͒ and P2͑x2, y2 ͒ is y L ⌬y y Ϫ y m ෇ ෇ 2 1 P™(x™, y™) ⌬x x2 Ϫ x1 Îy=fi-› The slope of a vertical line is not defined. P¡(x¡, y¡) =rise Îx=¤-⁄ ⌬ ⌬ =run Thus the slope of a line is the ratio of the change in y,y , to the change in x,x . (See Figure 7.) The slope is therefore the rate of change of y with respect to x. The fact that the 0 x line is straight means that the rate of change is constant. Figure 8 shows several lines labeled with their slopes. Notice that lines with positive FIGURE 7 slope slant upward to the right, whereas lines with negative slope slant downward to the right. Notice also that the steepest lines are the ones for which the absolute value of the y m=5 slope is largest, and a horizontal line has slope 0. m=2 Now let’s find an equation of the line that passes through a given point P1͑x1, y1͒ and m=1 has slope m. A point P͑x, y͒ with x x1 lies on this line if and only if the slope of the line 1 m=2 through P1 and P is equal to m; that is, y Ϫ y 1 ෇ m m=0 x Ϫ x1 1 m=_ 2 This equation can be rewritten in the form 0 m=_1 x Ϫ ෇ ͑ Ϫ ͒ m=_2 y y1 m x x1 m=_5 and we observe that this equation is also satisfied when x ෇ x1 and y ෇ y1. Therefore, it is FIGURE 8 an equation of the given line. Point-Slope Form of the Equation of a Line An equation of the line passing through the point P1͑x1, y1͒ and having slope m is y Ϫ y1 ෇ m͑x Ϫ x1͒ Thomson Brooks-Cole copyright 2007 4 ■ REVIEW OF ANALYTIC GEOMETRY EXAMPLE 3 Find an equation of the line through the points ͑Ϫ1, 2͒ and ͑3, Ϫ4͒. SOLUTION The slope of the line is Ϫ4 Ϫ 2 3 m ෇ ෇ Ϫ 3 Ϫ ͑Ϫ1͒ 2 Using the point-slope form with x1 ෇ Ϫ1 and y1 ෇ 2, we obtain Ϫ ෇ Ϫ 3 ͑ ϩ ͒ y 2 2 x 1 which simplifies to 3x ϩ 2y ෇ 1 y Suppose a nonvertical line has slope m and y-intercept b.

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