polar coordinates

can someone give me an example in poloar coordinates where if you allow negative r it adds to the graph? im trying to figure out what convention my graphic calculator follows, (fx-9850)

r = cos(2x)
If you allow negative r you'll get 4 leaves (as in the pic attached), if not you'll only get the ones to the left and right.

Edit: just noticed all the extra bits - ignore them, it's from a different question when I was explaining something

you, um dont get marked wrong for plotting negative values do you? because i do..

You plot them or get marked wrong for plotting them? I think with edexcel you don't need to plot them, but I'd be surprised if you lost marks for including them. You could always put the parts of the curve where r is negative in dashed lines (I think this is the convention) or label them as r<0