Lamé's equations - wrong notes given? Please help, exam on Monday!

ironlight

1. The problem statement, all variables and given/known data

This relates to using Lamé equations to find the stresses on a thick cylinder, but there are problems with our notes and the exam is on Monday.

For an exam question, the formula I've been given is that sigma r (radial stress) = a + b/r^2, but in the notes (http://www.tobynorris.com/work/prog/...derivation.htm) it appears that sigma r = a - b/r^2. I can't find an instance where it shows the same equation. Which one is correct? I understand that a is Pi a^2...etc and that b is (Pi-Po)...etc, but which way around is it?


2. Relevant equations

According to formula sheet in past exam paper:

sigma r = a + b/r^2
sigma c = a - b/r^2

According to notes:

sigma r = a - b/r^2
sigma c = a + b/r^2

3. The attempt at a solution

I can do the equations easily enough, but knowing which way around the plus and minus goes in relation to the radial or hoop stress is making it hard to say which is which in the exam. The question is worth around 35 out of 100 marks, quite considerable in a final year exam, so every point counts.