# I General question about math required to do david morins book

1. Jul 16, 2016

### RubinLicht

So i dont have issues with most of his problems, but once in a while a question that requires a ridiculous math trick that i had no way of knowing comes up and i just wonder if this is something you pick up from various places, or can attain through a focused study of mathematics. the question that prompted me to post this was this:
and solution:

I get completely lost once i get to "In taking the derivative, the former dependence requires finding the value of theintegrand at the x0 limit...". and then i can follow the solution again once you get to "the solution to this is -> exponential"

The problem i had with this is that i immediately knew that the solution would be an exponential with some dependance on l in the exponent, however, there was simply no way for me to reach the solution because i had to use what ever math tricks he used here.

background: i can comfortably do approximations with taylor series and binomial theorem, so I'm not completely new to the world of approximations.

tldr i would like to know where to gain the math knowledge for these kind of tricks

2. Jul 17, 2016

### wrobel

this is not a trick this is a solution to the differential equation $\rho'=-\rho$. There is a mathematical subject called "Ordinary Differential Equations"

3. Jul 17, 2016

### RubinLicht

"I get completely lost once i get to "In taking the derivative, the former dependence requires finding the value of theintegrand at the x0 limit...". and then i can follow thesolution again once you get to "the solution to this is -> exponential" "

Tldr I knew how to do that part, and you would know this is if you read my post again

4. Jul 17, 2016

### wrobel

Last edited: Jul 17, 2016
5. Jul 17, 2016

### RubinLicht

Yes but my understanding of it is very poor because we seemed to just skip over it in class. My friend also said to read about the fundamental theorem of calculus, so I'll do that when i get time and come back to this problem. Thanks!