typo example 2.5.2

The prose uses variables x and t but the sample code uses a and t.

I think this is a typo.

(confused me for half an hour - i'm a beginner!)

Something that has me very confused about this example is why the cancel tactic is allowed in the x <= 0 case. I don't see where it is being shown that x != 0 so that a division by zero is ruled out. The tactic works, so I'm obviously just missing something.

btw I'm having a ton of fun and learning a lot working through this book!