Thread: stuck on this one - first post

just joined so don't let me down

Can't sort this one:

y = 3x
2y(squared) - xy = 15

Tried substituting y = 3x and x = y/3. I only get half of the right answer using x = y/3 as y = -3 and x= -1. Can't seem to get the full right answer.

Any ideas

Becca.

Originally Posted by becca999

just joined so don't let me down

Can't sort this one:

y = 3x
2y(squared) - xy = 15

Tried substituting y = 3x and x = y/3. I only get half of the right answer using x = y/3 as y = -3 and x= -1. Can't seem to get the full right answer.

Any ideas

Becca.

Originally Posted by OReilly

Of course, also has the solution x = -1, but substitution shows that this x value does not provide a new intersection point.

-Dan

Originally Posted by topsquark

Of course, also has the solution x = -1, but substitution shows that this x value does not provide a new intersection point.

-Dan

Both solutions (1,3) and (-1,-3) are correct and they providing intersection points.

Graph is showing that.

Originally Posted by topsquark

Of course, also has the solution x = -1, but substitution shows that this x value does not provide a new intersection point.
-Dan

Hi,

actually you calculate the intersection between a hyperbola and a straight line. And of course there are two intersection points.
I've attached a diagram to show you where these points are:

Pffl on me. The second equation is . When I did the problem in my head for x = -1 I was using "+xy" Ah well. You win some, you lose some!

-Dan