I'm going call the stuff within the parentheses for A since it does not depend on t or D.
Then we have and
Comparing the two we see that
There's this equation that I have stumbled whilst working on several exercise questions.
I'm sorry that I post this in image, I've tried to write it down using Latex but it shows Error in preview mode.
The instruction is compare the derivative of P in respect to t with the derivative of P in respect to D. I have tried to solve it but my end calculation shows 9/DT instead of what it is supposed to be in the answer key, which is -D/T. I'm a bit confused here, what I'm doing wrong?