7-1-2

> restart;

> x:=-3*cos(t)+cos(-3*t):;

> y:=-3*sin(t)+sin(-3*t):;

> a:=plot([x,y,t=-Pi..Pi],color=red,scaling=constrained):

> a:;

> x1:=diff(x,t):;

> y1:=diff(y,t):;

> x2:=diff(x1,t):;

> y2:=diff(y1,t):;

> f:=x1^2+y1^2:;

> g:=x1*y2-y1*x2:;

> r:=f^(3/2)/abs(g):;

> xc:=x-f/g*y1:;

> yc:=y+f/g*x1:;

> m:=[[x,y],[xc,yc]]:

> t:=n*2*Pi/100+0.001:

> plot([m$n=0..100],scaling=constrained,color=blue,axes=none);

>