directrix of conic
In this case, a=2. To do this, take half the coefficient of x and square it. The graph of this ellipse appears as follows. /ExtGState A parabola is generated when a plane intersects a cone parallel to the generating line. stream Comparing this to Equation gives \(h=2, k=1\), and \(p=2\). Since \(c> If so, the graph is a hyperbola. /SMask 11 0 R Since then, important applications of conic sections have arisen (for example, in astronomy), and the properties of conic sections are used in radio telescopes, satellite dish receivers, and even architecture. Now. << Using the definitions of the focal parameter and eccentricity of the conic section, we can derive an equation for any conic section in polar coordinates. << In fact, in summer for the northern hemisphere, Earth is farther from the Sun than during winter. Another interesting fact about hyperbolas is that for a comet entering the solar system, if the speed is great enough to escape the Sun’s gravitational pull, then the path that the comet takes as it passes through the solar system is hyperbolic. If S is the focus and l is the directrix, then the set of all points in the plane whose distance from S bears a constant ratio e called eccentricity to their distance from l is a conic section. The right vertex of the ellipse is located at \((a,0)\) and the right focus is \((c,0)\). /Filter /FlateDecode \(4AC−B^2<0\). where A and B are either both positive or both negative. Parabolas have one Recall the distance formula: Given point P with coordinates \((x_1,y_1)\) and point Q with coordinates \((x_2,y_2),\) the distance between them is given by the formula, \[d(P,Q)=\sqrt{(x_2−x_1)^2+(y_2−y_1)^2}.\], Then from the definition of a parabola and Figure \(\PageIndex{3}\), we get, \[\sqrt{(0−x)^2+(p−y)^2}=\sqrt{(x−x)^2+(−p−y)^2}.\], Squaring both sides and simplifying yields, \[ \begin{align} x^2+(p−y)^2 &= 0^2+(−p−y)^2 \\ x^2+p^2−2py+y^2 &= p^2+2py+y^2 \\ x^2−2py& =2py \\ x^2& =4py. The parabola has an interesting reflective property. The axis of symmetry of a vertical (opening up or down) parabola is a vertical line passing through the vertex. The graph of this parabola appears as follows. Suppose, the angle formed between the surface of the cone and its axis is β and the angle formed between the cutting plane and the axis is α, the eccentricity is; Apart from focus, eccentricity and directrix, there are few more parameters defined under conic sections. Figure \(\PageIndex{15}\): Graph of the ellipse described by the equation \(13x^2−6\sqrt{3}xy+7y^2−256=0\). If 0≤β<α, then the plane intersects both nappes and conic section so formed is known as a hyperbola (represented by the orange curves). /CA 1 This gives the equation, We now define b so that \(b^2=c^2−a^2\). /AIS false First find the values of e and p, and then create a table of values. Going through the same derivation yields the formula \((x−h)^2=4p(y−k)\). This is a hyperbola opening up and down. After the introduction of Cartesian coordinates, the focus-directrix property can be utilised to write the equations provided by the points of the conic section. /Type /XObject The three conic sections with their directrices appear in Figure \(\PageIndex{12}\). The directrices of a horizontal hyperbola are also located at \(x=±\dfrac{a^2}{c}\), and a similar calculation shows that the eccentricity of a hyperbola is also e=ca. Figure \(\PageIndex{2}\): The four conic sections. If \(B≠0\) then the coordinate axes are rotated. Consider a fixed vertical line ‘l’ and another line ‘m’ inclined at an angle ‘α’ intersecting ‘l’ at point V as shown below: The initials as mentioned in the above figure A carry the following meanings: Let us briefly discuss the different conic sections formed when the plane cuts the nappes (excluding the vertex). This equation is therefore true for any point on the hyperbola. \(4AC−B^2=0\). The points \(Q\) and \(Q′\) are located at the ends of the minor axis of the ellipse, and have coordinates \((0,b)\) and \((0,−b),\) respectively. Asymptotes \(y=−2±\dfrac{3}{2}(x−1).\). Add the second radical from both sides and square both sides: \[\sqrt{(x−c)^2+y^2}=2a+\sqrt{(x+c)^2+y^2}\], \[(x−c)^2+y2=4a^2+4a\sqrt{(x+c)^2+y^2}+(x+c)^2+y^2\], \[x^2−2cx+c^2+y^2=4a^2+4a\sqrt{(x+c)^2+y^2}+x^2+2cx+c^2+y^2\], \((x+c)^2+y^2=a^2+2cx+\dfrac{c^2x^2}{a^2}\), \(x^2+2cx+c^2+y^2=a^2+2cx+\dfrac{c^2x^2}{a^2}\), Finally, divide both sides by \(a^2−c^2\). So, eccentricity is a measure of the deviation of the ellipse from being circular. Put the equation \(4y^2−9x^2+16y+18x−29=0\) into standard form and graph the resulting hyperbola. To determine the angle θ of rotation of the conic section, we use the formula \(\cot 2θ=\frac{A−C}{B}\). For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Conic sections are generated by the intersection of a plane with a cone (Figure \(\PageIndex{2}\)). >> For the conic [math]A{x^2} + Bxy + C{y^2 Similarly, we are subtracting 16 from the second set of parentheses. Figure \(\PageIndex{14}\): Graph of the equation \(xy=1\); The red lines indicate the rotated axes. /BBox [0 0 456 455] The equation for each of these cases can also be written in standard form as shown in the following graphs. endstream If it is on the left branch, then the subtraction is reversed. Therefore the distance from the vertex to the focus is \(a−c\) and the distance from the vertex to the right directrix is \(\dfrac{a^2}{c}−c.\) This gives the eccentricity as, \[e=\dfrac{a−c}{\dfrac{a^2}{c}−a}=\dfrac{c(a−c)}{a^2−ac}=\dfrac{c(a−c)}{a(a−c)}=\dfrac{c}{a}.\]. The conic sections in the Euclidean plane have various distinguishing properties, many of which can be used as alternative definitions. x���1  �O�e� ��� This value identifies the conic. >> The directrix of a conic section is the line that, together with the point known as the focus, serves to define a conic section. One such property defines a non-circular conic to be the set of those points whose distances to some particular point, called a focus, and some particular line, called a directrix, are in a fixed ratio, called the eccentricity. For the parabola, the standard form has the focus on the x-axis at the point (a, 0) and the directrix is the line with equation x = −a. >> To determine the rotated coefficients, use the formulas given above: \(=13\cos^260+(−6\sqrt{3})\cos 60 \sin 60+7\sin^260\), \(=13(\dfrac{1}{2})^2−6\sqrt{3}(\dfrac{1}{2})(\dfrac{\sqrt{3}}{2})+7(\dfrac{\sqrt{3}}{2})^2\), \(=13\sin^260+(−6\sqrt{3})\sin 60 \cos 60=7\cos^260\), \(=(\dfrac{\sqrt{3}}{2})^2+6\sqrt{3}(\dfrac{\sqrt{3}}{2})(\dfrac{1}{2})+7(\dfrac{1}{2})^2\), The equation of the conic in the rotated coordinate system becomes. The derivation of the equation of a hyperbola in standard form is virtually identical to that of an ellipse. /Height 1894 Figure \(\PageIndex{10}\): Graph of the hyperbola in Example. endobj \(4AC−B^2>0\). >> /FormType 1 The polar equation of a conic section with eccentricity. /Subtype /Image �0FQ�B�BW��~���Bz��~����K�B W ̋o << 10 0 obj /Length 63 Therefore \(2θ=120^o\) and \(θ=60^o\), which is the angle of the rotation of the axes. /XObject However in this case we have \(c>a\), so the eccentricity of a hyperbola is greater than 1. If 0≤β<α, the section formed is a pair of intersecting straight lines. >> In the equation on the left, the major axis of the conic section is horizontal, and in the equation on the right, the major axis is vertical. First find the values of a and b, then determine c using the equation \(c^2=a^2+b^2\). In particular, we assume that one of the foci of a given conic section lies at the pole. /Filter /FlateDecode The new coefficients are labeled \(A′,B′,C′,D′,E′,\) and \(F′,\) and are given by the formulas, \[ \begin{align} A′ &=A\cos^ 2θ+B\cos θ\sin θ+C\sin^2 θ \\ B′&=0 \\ C′&=A\sin^2 θ−B\sin θ\cos θ+C\cos^2θ \\ D′&=D\cos θ+E\sin θ \\ E′&=−D\sin θ+E\cosθ \\ F′&=F. Since y is not squared in this equation, we know that the parabola opens either upward or downward. The same thing occurs with a sound wave as well. /Type /XObject A conic section can also be described as the locus of a point P moving in the plane of a fixed point F known as focus (F) and a fixed line d known as directrix (with the focus not on d) in such a way that the ratio of the distance of point P from focus F to its distance from d is a constant e known as eccentricity.

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