Identify the substitution that transforms ∫ex1+e2xdx\int \frac{e^x}{1+e^{2x}} dx∫1+e2xexdx into an integral of the form ∫11+u2du\int \frac{1}{1+u^2} du∫1+u21du.
u = e^{2x}
u = e^x
u = 1+e^x
u = x^2