Well, the thread title is " Indefinite integral with continous funtions problem"; and in order to have the stated (continuous) derivative f(x), F(x)...
It sounds like you have never seen a derivation of the combination formula (or never paid attention). Rather than reinventing the wheel by trying to...
Are you aware of the shortcut for compound interest? Rather than calculate the interest as i = Prt and then adding that to P, you can combine that as...
I was a software engineer for many years, and have been teaching at a community college since 2004. Since 1998, I was a part of Ask Doctor Math (http://mathforum.org/dr.math/ and now http://themathdoctors.org/).