Given a line with slope y/x, find a simple formula for the slope of a second line that forms a 45 degree angle with this line (find slopes for both 45 degrees more and 45 degrees less.)
This can be done without trigonometry.
Find a general formula for any angle.
This requires trigonometry.
(In reply to
Solution by Bractals)
You lost me here. How would you use this formula for, say a 30 degree rotation?
Where does the 30 go?

Let p/q be the slope of the general angle V and y/x the slope
of angle U.
y p
 + 
tan(U) + tan(V) x q px + qy
tan(U+V) =  =  = 
1  tan(U)*tan(V) y p qx  py
1  *
x q

Posted by Jer
on 20060322 14:25:41 