The most general conic section has an equation of the form
Ax2+By2+Cxy+Dx+Ey+F=0.
So far we have taken C=D=E=0, except
for parabolas. Here we'll see how to adjust for nonzero values of D
and E. We'll still assume that C=0, which means that our curves
will point in the direction of the coordinate axes. C≠0
describes rotated conic sections.
If A≠0, then we can always absorb the Dx term by completing
the square:
Ax2+Dx=A(x2+DAx)=A(x+D2A)2−D24A
Likewise, if B≠0 then we can absorb
the Ey term:
By2+Ey=B(y+E2B)2−E24B.
If B=0 then we cannot absorb the Ey term, and the equation of a
parabola winds up looking like y−k=−AE(x−h)2.
Example : Identify the center, foci and
eccentricity of the ellipse 9x2+25y2−18x+50y−191=0.
Solution: Rewrite 9x2−18x as 9(x−1)2−9, and rewrite 25y2+50y as 25(y+1)2−25. This makes the equation
9(x−1)2+25(y+1)2−225=0,
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or equivalently
(x−1)225+(y+1)29=1.
This is an ellipse centered
at (1,−1) with a=5 and b=3, hence c=√52−32=4. The foci
are 4 units the the right and left of the center, at (5,−1) and
(−3,−1). This ellipse has eccentricity c/a=4/5=0.8.
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