math 54 lecture 6 - conic sections
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Math 54 Lecture 6 - Conic SectionsTRANSCRIPT
Introduction Parabola Ellipse Hyperbola Exercises
Conic Sections
Institute of Mathematics, University of the Philippines Diliman
Mathematics 54–Elementary Analysis 2
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Introduction Parabola Ellipse Hyperbola Exercises
Introduction
A conic section is a plane figure formed when a right-circular cone is cut by a plane.Below are samples of conic sections.
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Introduction Parabola Ellipse Hyperbola Exercises
Parabola
The set of all points in the plane whose distance from the fixed point (focus) equalsthe distance from the fixed line (directrix).
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Parabola
Consider the directix and the focus situated this way:
A point P = (x,y) is on the parabola ifdistance of P to F = distance of P to D√(x−p)2 + (y−0)2 = ∣∣x− (−p)
∣∣√(x−p)2 +y2 = ∣∣x+p
∣∣(x−p)2 +y2 = (x+p)2
x2 −2px+p2 +y2 = x2 +2px+p2
y2 = 4px
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Parabola
For p > 0,
Focus Directrix Equation Orientation
(p,0) x =−p y2 = 4px right
(−p,0) x = p y2 =−4px left
(0,p) y =−p x2 = 4py upward
(0,−p) y = p x2 =−4py downward
The point on the parabola closest to the diretrix is called the vertex of the parabola.In every cases consider above, the vertex is at the origin.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Parabola
In general, a parabola with a vertical or a horizontal directrix can take either of theform
(y−k)2 =±4p(x−h) (x−h)2 =±4p(y−k)
In these general cases, the vertex is located at the point (h,k).
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Parabola
For (y−k)2 = 4p(x−h) with p > 0,
Focus Directrix(p+h,k) x =−p+h(−p+h,k) x = p+h
For (x−h)2 = 4p(y−k) with p > 0,
Focus Directrix(h,p+k) y =−p+k(h,−p+k) y = p+k
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Parabola
Example
Find the focus, directrix and vertex of the parabola being described by theequation
y = 2x2 −4x+3
Solution:Note that the above equation can be written as
2(x−1)2 = (y−1)
which can be further written as
4
(1
2
)(x−1)2 = (
y−1)
.
Hence, the focus is at (1, 32 ), the directrix is y = 1
2 and the vertex is at (1,1).
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Introduction Parabola Ellipse Hyperbola Exercises
Ellipse
The set of all points in the plane whose distances from the fix points (foci sing.focus) sum up to a constant.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
Foci : F1 = (c,0) and F2 = (−c,0)
Vertices : V1 = (a,0) and V2 = (−a,0)
Endpoints of minor axis: (b,0) and (−b,0)
Point P = (x,y) is on the ellipse if
distance of P to F1 + distance of P to F2 = constant2a√(x+ c)2 +y2 +
√(x− c)2 +y2 = 2a
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
√(x+ c)2 +y2 = 2a−
√(x− c)2 +y2
x2 +2cx+ c2 +y2 = 4a2 −4a√
(x− c)2 +y2 +x2 −2cx+ c2 +y2
4a√
(x− c)2 +y2 = 4a2 −4cx
a2(x2 −2cx+ c2 +y2
)= a4 −2a2cx+ c2x2
a2x2 −2a2cx+a2c2 +a2y2 = a4 −2a2cx+ c2x2(a2 − c2
)x2 +a2y2 = a2
(a2 − c2
)x2
a2+ y2
a2 − c2= 1
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
x2
a2+ y2
a2 − c2= 1
Notice that b2 = a2 − c2.
Thus, we have
x2
a2+ y2
b2= 1.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
Forx2
a2+ y2
b2= 1 we have
Foci Vertices
a > b (±p
a2 −b2,0) (±a,0)
a < b (0,±p
b2 −a2) (0,±b)
The ellipses considered above are those centered at the origin.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
In general, we may consider an ellipse with either a vertical or a horizontal majoraxis centered somewhere aside from the origin. They take the form
(x−h)2
a2+ (y−k)2
b2= 1
Foci Vertices Major Axis
a > b (h±p
a2 −b2,k) (h±a,k) horizontal
a < b (h,k±p
b2 −a2) (h,k±b) vertical
In the cases considered above, the ellipse is centered at (h,k).
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of an Ellipse
Example
Find the equation of the ellipse centered at (1,1) with foci (−1,1) and (3,1) andvertices (−2,1) and (4,1).
Solution:The major axis of our ellipse is horizontal. From the data above, (h,k) = (1,1), a = 3,c = 2 and b =p
5.
Hence, we have
(x−1)2
9+ (y−1)2
5= 1
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Introduction Parabola Ellipse Hyperbola Exercises
Hyperbola
The set of all points in the plane whose distance from the two fixed points (foci)that have a constant difference.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
Foci : F1 = (c,0) and F2 = (−c,0)
Vertices : V1 = (a,0) and V2 = (−a,0)
From the above figure and from the definition of the hyperbola we have√(x+ c)2 +y2 −
√(x− c)2 +y2 = constant2a
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
Simplifying, we get
x2
a2− y2
c2 −a2= 1
Note that a < c. Thus, c2 −a2 > 0. Let b2 = c2 −a2. Then
x2
a2− y2
b2= 1
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
If we solve for y in terms of x we get
y =±√
x2b2
a2−b2
Consider the lines y = ba x and y =− b
a x. Verify the following:
limx→+∞
±√
x2b2
a2−b2
±b
ax
= 1
limx→−∞
±√
x2b2
a2−b2
∓b
ax
= 1
Hence, the lines y =± ba x serve as asymptotes of the hyperbola.
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
In general, for
x2
a2− y2
b2= 1
we have
a > b
Foci Vertices
(±√
a2 +b2,0) (±a,0)
a < b
Foci Vertices
(0,±√
a2 +b2) (0,±b)
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
In general, we may consider a hyperbola with either a vertical or a horizontaltransverse axis centered somewhere aside from the origin, say at (h,k). They takethe form
(x−h)2
a2− (y−k)2
b2= 1
a > b
Foci Vertices
(h±√
a2 +b2,k) (h±a,k)
a < b
Foci Vertices
(h,k±√
a2 +b2) (h,k±b)
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Introduction Parabola Ellipse Hyperbola Exercises
Equation of a Hyperbola
Example
Find the equation of the hyperbola with foci (−1,1) and (4,1) and vertices (0,1) and(3,1).
Solution:
The desired hyperbola is centered at ( 32 ,1) so h = 3
2 and k = 1.
The value of a is the distance from the center to a vertex, which in this case is 32 .
The value of c is the distance of a focus to the center which is inthis case is 52 .
Now, b2 = c2 +a2 = 164 = 4. Hence, the desired equation is
(x− 32 )2(
32
)2− (y−1)2
(2)2= 1
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Introduction Parabola Ellipse Hyperbola Exercises
Exercises
1 Identify the following conic sections:
a. 2x2 −3y2 +4x+6y−1 = 0b. 2x2 +3y2 +16x−18y−53 = 0c. 9x+y2 +4y−5 = 0d. 4x2 −x = y2 +1e. 7y−y2 −x = 0
2 Sketch the graph of the following conic sections:
a. x2 −2x+y = 0b. 9x2 −18x+4y2 = 27c. y = 3x2 −6x+5
d.y2
9− (x+2)2 = 1
e.(x−3)2
25− y2 +4y+4
9= 1
3 Do as indicateda. Determine the equation of the parabola whose focus and vertex are the vertex
and focus, respectively of the parabola with equation x+4y2 −y = 0.b. Let M = 3. Determine the equations of the hyperbola and ellipse having (±2,1) as
the foci and M as the length of the conjugate axis and minor axis, respectively.
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