You CAN Ace Calculus  

17calculus > conics  


Topics You Need To Understand For This Page
Calculus Main Topics
Single Variable Calculus 

MultiVariable Calculus 
Tools
math tools 

general learning tools 
additional tools 
Related Topics and Links
external links you may find helpful 

Conics 

on this page: ► parabola ► ellipse ► hyperbola 
A conic (or conic section) is a smooth curve formed when a plane intersects a pair of right circular cones placed pointtopoint. The angle of the plane measured with respect to the axis running through the point of the cones, determines the type of conic that is formed. 
Search 17Calculus 
There are three types of curves.
1. Parabolas
2. Ellipses (circles are special cases of ellipses and are sometimes listed as a fourth type)
3. Hyperbolas
Parabolas and hyperbolas are very similar and are easy to confuse. One difference is that there are a pair of curves in the case of a hyperbola but parabolas occur as a single curve.
The general equation for all these equations is \(Ax^2+Bxy+Cy^2+\) \(Dx+Ey+F=0\). There are a lot of differences in this equation for each curve. Let's discuss each type.
Parabola 
A parabola is formed when a plane intersections a cone parallel to the side of the cone. This intersection produces only one curve (as compared to two in a hyperbola).
Depending on the orientation of the cone with respect to the coordinate axis (xyaxis in 2 dimensions), the equations will be different. We will look only at the two cases where the coordinate axes runs parallel to the axis of the cone and perpendicular to the axis of the cone. These two cases will produce four possible parabolas.
1. Parabola opens up.
2. Parabola opens down.
3. Parabola opens to the left.
4. Parabola opens to the right.
We can combine types 1 and 2 to get one equation form and similarly, types 3 and 4 can be combined for a second equation form.
opens up or down  \( (xh)^2 = 4p(yk) \) 

opens left or right  \( (yk)^2 = 4p(xh) \) 
Of course, as with many mathematical equations, there are several ways to write this. However, written this way, we can directly pull a lot of information from it for the graph. (It is easy to convert from the form \(y=ax^2+bx+c\) for example that you are probably more familiar with, using some algebra and completing the square. )
We use special terms to describe parts of the parabola. You are probably familiar with the vertex. The other attributes are listed below (with equations from a parabola that opens up or down; you should be able to translate them to the other orientation) and shown in the figure on the right. Shown very well in this figure, are two pink lines with a black bar crossing them. The black bar indicates that the length of these lines are equal to one another. That is the definition that makes the parabola a parabola.
Classify   When looking at the equation in the form \(Ax^2+Bxy+Cy^2+\) \(Dx+Ey+F=0\), a parabola will have \(B=0\) and either \(A=0\) or \(C=0\) but not both.
\( (xh)^2 = 4p(yk) \)  

vertex  \((h,k)\) 
axis of symmetry  \(x=h\) 
focus  \((h,k+p)\) 
directrix  \(y=kp\) 
\(Ax^2+Dx+Ey+F=0\)  
Example \((x2)^2=4(y3)\)  
\( (yk)^2 = 4p(xh) \)  

vertex  \((h,k)\) 
axis of symmetry  \(y=k\) 
focus  \((h+p,k)\) 
directrix  \(x=hp\) 
\(Cy^2+Dx+Ey+F=0\)  
Example \((x2)=4(y3)^2\)  
Practice 1 

Sketch the graph of the parabola \(\displaystyle{ \frac{x^2}{12} = \frac{y}{3} }\). 
solution 
Practice 2 

Find the focus and the directrix of the parabola \(\displaystyle{ y = \frac{2}{5}x^2 }\). 
solution 
Practice 3 

Write the standard form of the equation for the parabola with the vertex at the origin and the focus at \((1/8,0)\). 
solution 
Practice 4 

Write the standard form of the equation for the parabola with the vertex at the origin and the directrix at \(y=5/6\). 
solution 
Practice 5 

Classify and list the attributes of the conic \(x^24x4y=0\). 
solution 
Ellipse 
An ellipse is formed when a plane crosses that is not parallel to one of sides of the cone and not parallel to the axis of the cone. A circle is a special form of an ellipse where the plane is perpendicular to the axis of the cone.
The standard equation for an ellipse is \(\displaystyle{ \frac{(xh)^2}{a^2} + \frac{(yk)^2}{b^2} = 1 }\). The plot on the right contains more information than we need right now but it will suffice. The longer axis is called the major axis (in this plot it is horizontal). The shorter axis is called the minor axis. The vertices are located on the ellipse where it crosses the major axis. The foci are also on the major axis, labeled F_{1} and F_{2} on this plot.
The major axis is determined by the denominators, \(a^2\) and \(b^2\). The larger value is in the denominator of the major axis, i.e. if \(a > b\) then the major axis is parallel to the xaxis. We need to define a value c where \(c^2=\abs{a^2b^2}\) which will help us determine the location of the foci.
These tables contain the main attributes of an ellipse. We assume here that \(a > b\). Similar equations exist for \(a < b\).
\(\displaystyle{ \frac{(xh)^2}{a^2} + \frac{(yk)^2}{b^2} = 1 }\)  

center  \((h,k)\) 
major axis  \(y=k\) 
vertices  \((h \pm a, k), \) \( (h, k \pm b)\) 
foci  \( (h \pm c,k) \) 
\(c^2=\abs{a^2b^2}\)  
eccentricity  \( e=c/a \) 
\(\displaystyle{ \frac{(xh)^2}{b^2} + \frac{(yk)^2}{a^2} = 1 }\)  

center  \((h,k)\) 
major axis  \(x=h\) 
vertices  \((h, k \pm a), \) \( (h \pm b, k)\) 
foci  \( (h,k \pm c) \) 
\(c^2=\abs{a^2b^2}\)  
eccentricity  \( e=c/a \) 
Notes
1. The eccentricity e is not the same as the irrational constant e. Since the foci are closer to the center than the vertices, it follows that \(c < a\) and therefore \(0 < e < 1\).
2. Notice in the standard form of the equation, both terms are positive. This is how you know the graph is an ellipse and not a hyperbola.
3. In the general form of the equation, \(Ax^2+Bxy+Cy^2+\) \(Dx+Ey+F=0\), \(A > 0\) and \(C > 0\).
Okay, time for some fun videos about ellipses. Here are a couple of videos about playing pool on an elliptical table. They clearly show the relationship between the foci and demonstrate some fun physics at the same time.
Numberphile  Elliptical Pool Table  
Practice 6 

Write the standard form of the equation for an ellipse, centered at the origin, vertical major axis of length 8 and minor axis of length 2. 
solution 
Practice 7 

Write the standard form of the equation for an ellipse, centered at the origin, with xintercepts at \(\pm 12\) and foci at \((0,\pm 5)\). 
solution 
Practice 8 

Write the standard form of the equation for an ellipse, centered at the origin, with minor axis of length 6 and foci at \(\pm 8, 0\). 
solution 
Practice 9 

Find the intercepts of the ellipse \(\displaystyle{\frac{y^2}{100}+\frac{x^2}{121}=1}\). 
solution 
Practice 10 

Graph the ellipse \(\displaystyle{1\frac{y^2}{16}=x^2}\). 
solution 
Practice 11 

Find the center and the radius of the circle \(x^2+2x+y^2=4\). 
solution 
Practice 12 

Sketch the circle \(x^2+y^2=4x\). 
solution 
Practice 13 

Sketch the circle \(x^2+y^2+6y=0\). 
solution 
Practice 14 

Sketch the circle \(x^2+y^2+2x+2y=2\). 
solution 
Practice 15 

Sketch the circle \(x^2+y^2+10x20y+100=0\). 
solution 
Practice 16 

Sketch the circle \(2x^2+2y^2+2x2y=1\). 
solution 
Practice 17 

Sketch the circle \(9x^2+9y^26x12y=11\). 
solution 
Practice 18 

Graph \(\displaystyle{\frac{x^2}{9}+\frac{y^2}{5}=1}\). 
solution 
Practice 19 

Graph \(4x^2+y^2=16\). 
solution 
Practice 20 

Graph \(x^2+2y^26x+4y+7=0\). 
solution 
Hyperbola 
The hyperbola is the most complicated of the three and, consequently, the most interesting. Again, the plot on the right contains more information than we need right now, but it will suffice.
The vertices (\(\pm a\) on this plot) and the foci (F_{1} and F_{2} on this plot) lie on the transverse axis with the center at \((h,k)\). The standard equations are
\(\displaystyle{ \frac{(xh)^2}{a^2}  \frac{(yk)^2}{b^2} = 1 }\) 
horizontal transverse axis 
\(\displaystyle{ \frac{(yk)^2}{a^2}  \frac{(xh)^2}{b^2} = 1 }\) 
vertical transverse axis 
As we did with the ellipse, we define an intermediate value \(c^2=a^2+b^2\) which will help us locate the foci.
For a hyperbola, we need to know the equations of the lines in blue on the plot. These are asymptotes.
For a horizontal transverse axis, the asymptotes are \(\displaystyle{ y=k \pm \frac{b}{a}(xh) }\).
For a vertical transverse axis, the asymptotes are \(\displaystyle{ y=k \pm \frac{a}{b}(xh) }\).
Similar to the ellipse, we define the eccentricity as \(e=c/a\). The results are summarized next.
\(\displaystyle{ \frac{(xh)^2}{a^2}  \frac{(yk)^2}{b^2} = 1 }\)  

horizontal transverse axis 

center  \((h,k)\) 
vertices  \((h \pm a, k)\) 
foci  \( (h \pm c,k) \) 
asymptotes  \(\displaystyle{ y=k \pm \frac{b}{a}(xh) }\) 
\(c^2=a^2+b^2\)  
eccentricity  \( e=c/a \) 
\(\displaystyle{ \frac{(yk)^2}{a^2}  \frac{(xh)^2}{b^2} = 1 }\)  

vertical transverse axis 

center  \((h,k)\) 
vertices  \((h, k \pm a)\) 
foci  \( (h,k \pm c) \) 
asymptotes  \(\displaystyle{ y=k \pm \frac{a}{b}(xh) }\) 
\(c^2=a^2+b^2\)  
eccentricity  \( e=c/a \) 
Notes
1. Notice that the value of c is different here than for an ellipse.
2. Since \(c > a\), \(e > 1\).
The hyperbola is quite a complicated graph with lots of features. Here is a video that goes into more detail.
MIP4U  Conic Sections: The Hyperbola part 1 of 2  
Okay, time for some practice problems on hyperbolas. Although it may seem like we have given you a lot of information on this page, we have just skimmed the surface in the discussion of these three figures, especially the hyperbola. There are a lot more interesting features that we hope you get to explore in your class.
Practice 21 

Sketch the graph of the hyperbola \(\displaystyle{ (y+4)^2  \frac{x^2}{25} = 1 }\). 
solution 
Practice 22 

Graph \(\displaystyle{\frac{x^2}{49}\frac{y^2}{25}=1}\). 
solution 
Practice 23 

Write the equation of the hyperbola that has vertices \((2,5), (4,5)\) and foci \((4,5), (6,5)\). 
solution 
Practice 24 

Graph \(\displaystyle{ \frac{(x2)^2}{4}  \frac{(y+3)^2}{9} = 1 }\). 
solution 
Practice 25 

Graph \(\displaystyle{ \frac{(y+4)^2}{4}  \frac{(x2)^2}{16} = 1 }\). 
solution 
Practice 26 

Graph \(4x^2y^2=16\). 
solution 
Practice 27 

Graph \(x^2+4y^22x16y+11=0\). 
solution 