now I'm embarrassed ! Thanks Doug.

restart;
ross_x:=diff(x(t),t)=-y(t)-z(t):
ross_y:=diff(y(t),t)=x(t)+a*y(t):
ross_z:=diff(z(t),t)=b+x(t)*z(t)-c*z(t):
rossler_sys:=ross_x,ross_y,ross_z:
DEtools[DEplot3d]({rossler_sys},{x(t),y(t),z(t)},t=50..200,[[x(0)=1,y(0)=1,z(0)=1]],scene=[x(t),y(t),z(t)],stepsize=0.05,thickness=1,linecolor=blue,orientation=[40,120]);
I have created a worksheet with just the above commands (mw and mws)
On my system it results in a lost kernel connection, and then a corrupt worksheet using both mw and mws worksheets.
I know that the cause of the problem is that a, b and c are not assigned values when DEplot3d is called, but it doesn't seem right that this then causes Maple to lose it's kernel connection and corrupts the worksheet if I attempt to save it.
:(

OK, so I found that the cause of the problem of Maple "losing the connection to the kernel" was that a variable in my ODE system was unassigned. Strange, and not convenient !

Thanks for helping!!
Hmm, it seems I'm using standard gui, with the option for default format for new worksheet set to worksheet not document.
We have to submit mws files to our tutor for assignments and we were advised to use classic mode.
I originally had maple 11 supplied as part of my course, and it had an option "classic worksheet maple 11" in the menu, but I had some problems with it (64 bit issues) and since upgrading to maple 13 I don't have a "classic worksheet maple 13" option in the menu, just "command line maple 13" and "maple 13"..contrary to what is says here:
http://www.maplesoft.com/support/help/AddOns/view.aspx?path=versions
Removing the output doesn't seem to affect the "lost kernel connection" problem.
Thanks again.

Knew it had to be simple :)

Surely if x=0 then sin(x*y)=0 which contradicts the original relation sin(x*y)=1
From the original relationship, sin(x*y) = 1, provided that x is *not* equal to zero. Thus, xy = pi/2 ± k.2pi; ie xy = const. This gives dy/dx = - y/x immediately. In the second method, before dividing by x^2cos(x*y), we have the expression
x{cos(xy)[y + x.dy/dx]} + sin(xy) = 1. But sin(xy) = 1, xy = Pi/2 ±k*2*Pi, so cos(xy) = 0

I realised a little while ago that the question is quite strange. Adding to the strangeness is the page where I saw the question posed:
http://www.analyzemath.com/calculus/Differentiation/implicit.html
Note the given answer at the bottom

Yes, that's the document I mean and it crashes for me in classic mode, but works fine in standard gui.
Anyway, I'll just use it in standard gui mode.
One of the problems I have with standard gui is that I get messages such as "waiting for a connection to the maple kernel" at irregular intervals and in general it seems quite slow in comparison. Also, the course I'm taking specifies that we use classic mode.
Thanks anyway !

Surely an image/text capture module in the registration process along with email confirmation would put a halt to this ? I know that major spammers farm out their activities to armies of cheap overseas workers but that is highly unlikely to be the case here.
Contrary to the opinion of the 2nd poster in this topic, I don't see why this should be considered onerous on legitimate members.