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# Conics

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6 Jun 7, 2019

#1
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First one

-16x  = y^2      this is a parabola  opening to the left.....we have the form

4p ( x - h)  =  (y - k)^2

4p ( x - 0)  = (y - 0)^2

The vertex is at  (h, k)  = (0,0)

We can find "p" [ the distance between the focus and the vertex ] as

4p = - 16       divide both sides by 4

p = - 4

The focus is given by

(0 + p, 0)  =  (0 + (-4), 0)  =  (-4, 0)

The directrix is given by

x =  - (p)

x= - (-4)

x  = 4

Here's a graph :  https://www.desmos.com/calculator/cdaauapqz1   Jun 7, 2019
#2
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Second one

x^2          y^2

___   -     ____   =  1

25             1

We have the form

(x - h)^2           (y -k)^2

______   -      ________   =       1

a^2                  b^2

This is a hyperbola intersecting the x axis  with a center  at (h, k)  = (0, 0)

a = 5      and b  = 1

Tve vertices   lie    along the x axis  and  are given by  (a, 0)  and (-a, 0)  = (5, 0) and (-5, 0)

The foci  are given by   ( √[ a^2 + b^2], 0 )    and (- √[a^2 + b^2], 0 )   =

( √ [25 + 1] , 0 )   =  (√26, 0)       and  ( - √26, 0)

The asypmtotes are given by :

y = ± (b/a) (x - h) + k     =

y  = ± (1/5) ( x - 0) + 0    =

y =  ± (1/5)x

Here's a graph:  https://www.desmos.com/calculator/aabdp29eok   Jun 7, 2019
#3
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Last one

x^2         y^2

___  +   ____  =   1

25          16

The " + "  between the terms signals an ellipse

Like the hyperbola before, this is centered  at the origin = (0, 0)

The major axis is along x  and the minor along y

a^2  = 25       b^2  = 16

a = 5     b  =  4

The vertices here are located at   ( 5, 0) , (-5, 0), (0, 4) and (0, -4)

The foci are located on the major axis and  are given by  ( ±√[ a^2 - b^2 ] , 0) =

( ±√[ 25 - 16 ] , 0 )   =   (± √9 , 0 )   =  (3, 0) and (-3, 0)

Here's a graph :  https://www.desmos.com/calculator/j579zix3zu   Jun 7, 2019
#4
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BTW....here's a resource that I find helpful :

https://www.purplemath.com/modules/index.htm   Jun 7, 2019
#5
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Thank you sooo much CPhill these really helped me. One question though. For the second question, is that formula for asymptotes applicable to all hyperbolas or just this one specifically?

#6
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Whenever the hyperbola intersects the x axis  [like the one we did ]

The equation of the asymptotes  is

y  = ± (b/a) ( x - h) + k       where (h, k) is the center of the hyperbola

If the hyperbola intersects the y axis we have the form

(y - k)^2        ( x - h)^2

______    -   ________   = 1

a^2                  b^2

And the equation of the asymptotes is

y  =  ± (a/b) (x - h) +  k     where (h, k)  is the center   Jun 7, 2019