Posted by lyne on Saturday, October 24, 2009 at 9:29pm.
You'd recognise it. :-)
xy < 0 means xy is negative.
When x is 1, what values can y have? When x is 5? pi? 1,000,000,000?
When x is -1, what values can y have?
If the product of two numbers, say x * y, is negative, what do you know about those numbers? One is p------- and the other is n-------
So the graph of xy<0 is all the set of (x,y) pairs where x is p----- and y is n------ plus the set of (x,y) pairs where x is n----- and y is p------.
Note that neither x or y can be zero; that means the graph doesn't touch the axes.