Submitted by kingcrepe on Sat, 05/26/2018 - 17:58
Forums:
Say r=cos2(theta). When the angle is between pi/4 and 3pi/4 r will be negative, and the same between -pi/4 and -3pi/4. However when sketching the graph on desmos, and looking at everyone else's solutions to question 4 STEP 3 1998 the graph exists in these regions. But both in my further maths a level and the markscheme for STEP 3 2015 question 3, it states that r cannot be negative. So in summary can r be negative or not?
Negative r values in polar
Negative r values in polar coordinates *
Conventions
It's basically different conventions. A levels appear to be using the convention that $r \ge 0$, and this will be stated in the new STEP specs as well. I would use the convention $r \ge 0$ (and also state that you are using this!).
Thank you!
Thank you!