integrate e^(3x)
Let's integrate: ∫(e(3x))dx
We attempt to reverse an integration: starting from e(3x) and (exp(a))' = exp(a), we differentiate using exp
Let's perform the differentiation: (exp(3x))'
Apply the differentiation rule: (exp(f))' = (f)'·exp(f) with f = 3x
(3x)'·exp(3x)
3exp(3x)
You observe that the computed derivative and the original math expression are the same, except for a coefficient. So the integral is:
13·e(3x)
e(3x)3
Indefinite integrals are defined up to an additive constant C, so this yields: e(3x)3 + C
Result:   e(3x)3 + C, where C is a constant