Pranav from Vardhana School had a good
suggestion : copy the image from the NRICH webpage into Paint and
then make a copy you can rotate and compare.
Also if you copy into Word, and rotate a
copy, you could use one of the drawing tool to trace over one of
the curves you want to compare and then move that new line around
to see if it fits over the rotated copy.
Trace 2 does not line up with itself upside down. This can be
spotted by looking at the lowest point on the line, the curve is
sharper than at the highest point on the line, and so when rotated
these two can not line up.
Trace 3 on the other hand does line up with itself upside down.
This can be spotted by the fact that when you rotate the graph and
slide it along it lines up. Another method is to draw a line at the
You can now notice that when the wheel has turned 180 degrees, then
the rotated diagram is the same as the one above, and so the
rotated graph does line up with itself.