Question on Human Capital Accumulation Model by Acemoglu/Autor

Question

I was reading the lecture notes by Acemoglu and Autor on Labour Economics. https://economics.mit.edu/files/4689

I was able to understand equations 1.1 to 1.8 of the notes; however, I am unable to understand how equations 1.9 and 1.10 were derived from equations 1.8 and 1.9 respectively. I understand that integration and differentiation was applied, but I am not able to figure out the exact steps involved.

For the transition from equation 1.8 to 1.9, I tried to understand in terms of general power rule and general exponential rule of integration, but there are so many terms involved that I am unable to think how these rules apply.

For the transition from 1.9 to 1.10, I tried to understand in terms of general product rule and quotient rule of differentiation, but I am unable to figure out exact steps.

I would deeply appreciate if anyone could help me understand these steps. Please let me know if any other info is required from me.

Thank you.

Answer 1

The FOC from (1.9) is (I'm skipping the content of the $\exp()$ expression for brevity):

$ \frac{1}{r+\nu+g_h + g_w} \left(\eta'(S)w(0) \exp(\cdot) + \eta(S) w(0) (r + \nu - g_w)\exp(\cdot)\right) \overset{!}{=} 0$

This makes use of product rule. Since the first term can't be 0 due to the last equation on p.~11, it must be second one

$ \eta'(S)w(0) \exp(\cdot) = \eta(S) w(0) (r + \nu - g_w)\exp(\cdot) $

$ \eta'(S) = \eta(S) (r + \nu - g_w) $ yielding Eq. (1.10):

$ \frac{\eta'(S)}{\eta(S)} = r + \nu - g_w $