math
posted by lyne .
what would the graph of this look like
xy<0

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.