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College Algebra, 2013a

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7.4 Ellipses 517<br />

Minor Axis<br />

Major Axis<br />

V 1<br />

V 2<br />

F 1 C F 2<br />

An ellipse with center C; fociF 1 , F 2 ; and vertices V 1 , V 2<br />

Note that the major axis is the longer of the two axes through the center, and likewise, the minor<br />

axis is the shorter of the two. In order to derive the standard equation of an ellipse, we assume that<br />

the ellipse has its center at (0, 0), its major axis along the x-axis, and has foci (c, 0) and (−c, 0)<br />

and vertices (−a, 0) and (a, 0). We will label the y-intercepts of the ellipse as (0,b)and(0, −b) (We<br />

assume a, b, andc are all positive numbers.) Schematically,<br />

y<br />

(0,b)<br />

(x, y)<br />

(−a, 0) (−c, 0) (c, 0) (a, 0)<br />

x<br />

(0, −b)<br />

Note that since (a, 0) is on the ellipse, it must satisfy the conditions of Definition 7.4. Thatis,the<br />

distance from (−c, 0) to (a, 0) plus the distance from (c, 0) to (a, 0) must equal the fixed distance<br />

d. Since all of these points lie on the x-axis, we get<br />

distance from (−c, 0) to (a, 0) + distance from (c, 0) to (a, 0) = d<br />

(a + c)+(a − c) = d<br />

2a = d

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