find the vertices, center, and foci of the ellipse, and sketc its graph 9x^2+4y^2-36x+8y+31=0
Rearrange
Factor out coefficients of squared letters:
Complete the square in the first parentheses by
adding inside the first parentheses
which actually amounts to adding 36 to the left side
because there is a 9 in front of the parentheses, so
we must add a 36 to the right side:
Complete the square in the second parentheses by
adding inside the second parentheses
which actually amounts to adding 4 to the left side
because there is a 4 in front of the parentheses, so
we must add a 4 to the right side:
Factoring the parentheses as a perfect squares:
Get a 1 on the right by dividing through by 9
To get the 4 off the top of the second fraction we
divide top and bottom by 4:
Since the largest denominator is under the term in
y, the ellipse has a vertical major axis. So we
compare it to:
, ,
so
so
Its center is at (h,k) = (1,
Draw the major axis units both above and below the center.
Draw the minor axis units both right and left of the center.
The vertices are units above and below the center (2,-1)
So we add to the y-coordinate of the center
so the upper vertex is (2,)
And we subtract from the y-coordinate of the center
so the lower vertex is (2,)
Sketch in the ellipse:
To find the foci, we must calculate c, using the Pythagorean
relationship
The foci are units above and below the center (2,-1)
So we add to the y-coordinate of the center
so the upper vertex is (2,)
And we subtract from the y-coordinate of the center
so the upper vertex is (2,)
They are approximately
(1,0.12) and (1,-2.12)
We draw them in
Edwin