Another New Years' resolution

Because I'm not taking a formal mathematics class this semester, I've written a resolution that says I will read through G.H. Hardy's A Course of Pure Mathematics and do all the exercises.

I'm already finding (what I think are) mistakes. Here's a selection from page 5:

If the length PQ, along a given line, is a, and the length QR, along the same straight line, is b, the length PR must be (a+b).

To which I say,

I suppose that if you allow for negative lengths, then the rule still applies, but I've never heard of a negative length before. (For you physics freaks out there, yes, you can have a negative displacement, but this is a length, goddamnit.) I'm surprised to find what looks like a mistake in the tenth edition of a book that's supposed to be "formal and rigorous" (according to the Amazon reviewers). Perhaps it's because of a shift in terminology?