It is ambiguous, that's the trick. One can interpret "..." as '"any number of 9s", and in this interpretation the equation is false. But the teacher will eventu

It is ambiguous, that's the trick. One can interpret "..." as '"any number of 9s", and in this interpretation the equation is false. But the teacher will eventually ask, "what about an infinite number of 9s"? Bam! brilliant introduction to series and convergence. The correct notations to express exactly what one means feature the infinity symbol [1] - but if you use them, well, of course the students will see the trap. [1] https://en.wikipedia.org/wiki/Infinity#Real_analysis https://en.wikipedia.org/wiki/Infinity#Real_analysis