kartelite wrote:
Because you are getting a constant change in ratio, by definition the value of the derivative must be proportional to the value of the function at all times, hence d/dx(e^x) = e^x.
You took a good stab at it, but I had trouble understanding what you were trying to write in your last paragraph. It's hard to explain mathematical concepts in English. But, the last sentence you wrote doesn't make sense unless you replace "proportional" to "equal", because d/dx(a^x)=ln(a)*a^x, which is proportional to a^x.
Anyhow, there are two definitions of e, the compound interest one, and the calculus one (e^x is the function f such that df/dx = f), and you can prove that they're the same number.
Some of my family friends were complaining how hard it was to memorize all of their combinations (for locks, passwords, credit cards, etc), so I told them I sometimes just use numbers I know (obviously for passwords you don't care about) like e, pi, the square root of two or three... at which point I was stopped and asked what e is.