# Conic Sections

• Jul 7th 2009, 04:49 AM
Stroodle
Conic Sections
Hi.

I'm just starting to learn about conic sections, and it says in my text that their general equation is $a^2+by^2+2gx+2fy+c=0$.

Is that an error? Isn't it supposed to be $ax^2$ not just $a^2$ ?

• Jul 7th 2009, 04:54 AM
yeongil
Yes, the x-squared is missing. Also, the xy term is missing. In my book the general form of a quadratic equation in 2 variables is
$Ax^2 +Bxy +Cy^2 + Dx + Ey + F = 0$.

After a Google Search, I have found this equation also expressed as
$ax^2 + 2hxy + by^2 + 2gx + 2fy + c = 0$,
which is closer to what you have.

01
• Jul 7th 2009, 05:03 AM
Stroodle
Awesome. Thanks for that.

Would the $xy$ term be present for hyperbolas and ellipses also? As my text says that this general equation is for all conic sections...
• Jul 7th 2009, 05:19 AM
yeongil
Quote:

Originally Posted by Stroodle
Awesome. Thanks for that.

Would the $xy$ term be present for hyperbolas and ellipses also? As my text says that this general equation is for all conic sections...

Yes.

Actually, now that I think about it, if there is an xy term then the conic is rotated by some angle. You stated that you're starting to learn about conic sections, so the book probably omitted the xy term deliberately. This means that in parabolas & hyperbolas, without the xy term, the curves will only open upward/downward/left/right. And in ellipses, the major/minor axes will only be horizontal/vertical lines.

01
• Jul 7th 2009, 05:22 AM
Stroodle