# conics

• April 16th 2013, 10:49 PM
kastamonu
conics
Compute the points of the intersection between the circle x^2+y^2=1 and hyperbola xy=1?

I tried to put 1/y in place of x but I couldn't solve.
• April 16th 2013, 11:02 PM
Gusbob
Re: conics
Quote:

Originally Posted by kastamonu
Compute the points of the intersection between the circle x^2+y^2=1 and hyperbola xy=1?

I tried to put 1/y in place of x but I couldn't solve.

You have $x^2+y^2=1$ and $xy=1$.

We may assume $x,y\not=0$ since they don't lie on the hyperbola anyways.

Taking $x=1/y$ as you have tried,

$\frac{1}{y^2}+y^2=1$

Multiplying by $y^2$ and rearranging gives

$y^4-y^2+1=0$

which has no real solutions. So there is no intersection. In fact, this is obvious if you graphed the two functions.
• April 17th 2013, 02:58 AM
kastamonu
Re: conics
There is an answer. By discriminant we can find the roots. t^2=x^4 and we can find the roots.
\Delta = \,b^2-4ac.
• April 17th 2013, 03:17 AM
Gusbob
Re: conics
Yes, but the discriminant is negative, so all the roots are non-real numbers in the complex plane. That is to say there is no real solution. As I have suggested earlier, just graphing these two functions makes it clear that they do not intersect on the xy plane.
• April 17th 2013, 04:06 AM
MINOANMAN
Re: conics
• April 17th 2013, 09:20 AM
kastamonu
Re: conics
It is positive.
• April 17th 2013, 09:20 AM
kastamonu
Re: conics
good drawing.
• April 17th 2013, 09:21 AM
kastamonu
Re: conics
Quote:

Originally Posted by Gusbob
Yes, but the discriminant is negative, so all the roots are non-real numbers in the complex plane. That is to say there is no real solution. As I have suggested earlier, just graphing these two functions makes it clear that they do not intersect on the xy plane.

It is positive.
• April 18th 2013, 08:34 PM
ibdutt
Re: conics
The equation reduces to t^2-t+1=0 when we put t = y^2.
Now discriminant of equation is b^2-4ac= (1)^2- 4*1*1= 1 - 4 = -3 NEGATIVE
• April 19th 2013, 04:35 AM
kastamonu
Re: conics
ıt was x^2+y^2=4.I made a mistake.
• April 19th 2013, 09:02 AM
MINOANMAN
Re: conics
Kastamonu ...........