Find the integral that includes many level exponents
Solution 1:
Notice that you can write the integrand as $e^{-x}\cdot e^{-e^{-x}}$. Then use the substitution $u = -e^{-x}$. The rest is a straightforward calculation.
Notice that you can write the integrand as $e^{-x}\cdot e^{-e^{-x}}$. Then use the substitution $u = -e^{-x}$. The rest is a straightforward calculation.