# Ellipse Major Axis Rotation

1. Aug 25, 2010

### rapids79

Hi..I have a basic question regarding the equation of an ellipse. Lets say I ahve an ellipse with major and minor axes 2a and 2b respectively. Now, to check whether a point lies inside this ellipse, its fairly simple...I can just use the standars ellipse equation for that. Now, if my major axis is rotates to..lets say 45 degrees, how does the equation of the ellipse vary and how do I find then whether a certain point lies within the ellipse?? Logically, rotation of the major axis must not change the way I look for a point to be located inside/outside an ellipse. except that lets say once the rotation angle is >= 90 degrees your major axis becomes minor an vice-versa. Is my thinking correct?? or does the equation and method vary??? I'd really appreciate it if someone can throw me some pointers...thanks..

PS: Before any1 asks, this is not a homework problem..am a workign professional..cheers!!!

2. Aug 25, 2010

### quasar987

A point is part of the 45 degree-rotated ellipse <==> the -45 degree-rotated point is part of the nonrotated ellipse. Does that help?

What are you a working professional of, might I inquire?

3. Aug 25, 2010

### rapids79

Hi..Thanks for the reply..am just starting out in software...need to implement a code that does that for a project...so, anyway...if I have a point A...how would I check if it lies within the rotated ellipse...is there any mathematical method like the tangential method that does that???

4. Aug 25, 2010

### quasar987

According to the idea of post #2, you need to show that the point A, when rotated by -45 degree, is in the non rotated ellipse. Do you see that?

If so, then all you need is an equation for the coordinates of a point rotated by -45 degrees. The function R^2-->R^2 which rotates every point by an angle O is linear and its matrix is called a rotation matrix. Check out wikipedia. Applying this matrix on the left to a column vector (x y) will give you a column vector whose entries are the x and y coordinates of the rotated point. Use O=-pi/4 in this formula to get the result for the ellipse rotated by 45 degrees.