#### You may also like ### Quartics

Investigate the graphs of y = [1 + (x - t)^2][1 + (x + t^)2] as the parameter t varies. ### Sine Problem

In this 'mesh' of sine graphs, one of the graphs is the graph of the sine function. Find the equations of the other graphs to reproduce the pattern. ### Cocked Hat

Sketch the graphs for this implicitly defined family of functions.

# Maltese Cross

##### Age 16 to 18 Challenge Level:

The graph of points $(x,y)$ satisfying the equation

$$xy(x^2 - y^2) = x^2 + y^2$$

consists of four curves together with a single point at the origin.
You can use graphing software to sketch this graph but it is more of a challenge to see if you can sketch it for yourself and the steps in this question are designed to help you to do so. You can download the shareware program Graphmatica from http://www.graphmatica.com/

(a) Prove that the polar equation of this graph is

$$r^4 \sin 4\theta = 4r^2.$$

(b) Deduce that there are 4 points on this graph at distance 2 from the origin and no points closer to the origin. Find the values of $\theta$ for which there are points on the graph and the values of $\theta$ for which there are no points on the graph.

(c) Substitute $y=px$ in the Cartesian equation and find an expression for $x^2$ in terms of $p$. Hence find the values of $p$ for which the lines $y=px$ do not cut the graph in points other than the origin. For other values of $p$, in how many points do the lines $y=px$ cut the graph?

(d) Prove that if the point $(a,b)$ lies on the graph then so do the points $(-a,-b)$, $(-b,a)$ and $(b,-a)$.

(e) What can you say about the symmetries of the graph.