How do you find all the critical points to graph x^2 - 9y^2 + 2x - 54y + 80 = 0x29y2+2x54y+80=0 including vertices, foci and asympotes?

1 Answer
Oct 19, 2016

Please see the explanation.

Explanation:

Add h^2 - 9k^2 - 80h29k280 to both sides:

x^2 + 2x + h^2 - 9y^2 - 54y - 9k^2 = h^2 - 9k^2 - 80x2+2x+h29y254y9k2=h29k280

Set the right side of the pattern (x - h)^2 = x^2 - 2hx + h^2(xh)2=x22hx+h2 equal to the terms x^2 + 2x + h^2x2+2x+h2:

x^2 - 2hx + h^2 = x^2 + 2x + h^2x22hx+h2=x2+2x+h2

Solve for hh and h^2h2:

-2hx + h^2 = 2x + h^22hx+h2=2x+h2

-2hx = 2x2hx=2x

h = -1h=1 and h^2 = 1h2=1

Substitute the left side of the pattern (along with the value of h) for the 3 terms on the left and substitute 1 for h^2h2 on the right:

(x - -1)^2 - 9y^2 - 54y - 9k^2 = 1 - 9k^2 - 80(x1)29y254y9k2=19k280

Factor -9 from the y terms:

(x - 1)^2 - 9(y^2 + 6y + k^2) = 1 - 9k^2 - 80(x1)29(y2+6y+k2)=19k280

Set the right side of the pattern (y - k)^2 = y^2 - 2ky + k^2(yk)2=y22ky+k2 equal to the terms y^2 + 6y + k^2y2+6y+k2:

y^2 - 2ky + k^2= y^2 + 6y + k^2y22ky+k2=y2+6y+k2

Solve for kk and k^2k2:

-2ky= 6y2ky=6y

k = -3k=3 and k^2 = 9k2=9

Substitute the left side on the pattern (along with the value of k) into the ()s on the left and 9 for k^2k2 on the right:

(x - -1)^2 - 9((y - -3)^2) = 1 - 9(9) - 80(x1)29((y3)2)=19(9)80

Simplify the right side:

(x - -1)^2 - 9((y - -3)^2) = -160(x1)29((y3)2)=160

Divide both sides by -160:

9((y - -3)^2)/160 - (x - -1)^2/160 = 19(y3)2160(x1)2160=1

Write denominators as squares:

(y - -3)^2/((4sqrt(10))/3)^2 - (x - -1)^2/(4sqrt10)^2 = 1(y3)2(4103)2(x1)2(410)2=1

Center:(-1,-3)(1,3)
To find the vertices, make the negative term disappear by setting x = -1x=1:

(y - -3)^2/((4sqrt(10))/3)^2 = 1(y3)2(4103)2=1

(y - -3)^2 = ((4sqrt(10))/3)^2(y3)2=(4103)2

y - -3 = (4sqrt(10))/3y3=4103 and y - -3 = -(4sqrt(10))/3y3=4103

y = -3 + (4sqrt(10))/3y=3+4103 and y = -3 -(4sqrt(10))/3y=34103
Vertices:(-1, -3 + (4sqrt(10))/3)(1,3+4103) and (-1, -3 -(4sqrt(10))/3)(1,34103)

To find the focal distance, c, take the square root of the sum of the squares of the denominators:

c = sqrt(160/9 + 160)c=1609+160

c = sqrt(160/9 + 160)c=1609+160

c = 40/3c=403

The foci are at : (-1, -3 + 40/3)(1,3+403) and (-1, -3 - 40/3)(1,3403)

Foci:(-1, 31/3)(1,313) and (-1, - 49/3)(1,493)

To find the asymptotes, please observe that dividing the first denominator by the second gives the slopes of 1/313 and -1/313. All that is left to do is to force the lines with those two slopes, through the center point.

y = 1/3(x - -1) - 3y=13(x1)3
y = -1/3(x - -1) - 3y=13(x1)3