Using U substitution, find the integral of e^x e^(3x) dx.?

Answer 1

Let u = e^x, then e^(3x) = u^3 and du = e^x dx.
So ∫e^x e^(3x) dx = ∫u^3 du
= u^4 / 4 + c
= e^(4x) / 4 + c.