Evaluating a Definite Integral In Exercises $67-70,$ evaluate the definite integral.

$67.{\displaystyle \underset{0}{\overset{1}{}}}x{e}^{-3x^2}dx$

$68.{\displaystyle \underset{\frac{1}{2}}{\overset{2}{}}}\frac{e^{\frac{1}{x}}}{x^2}dx$

$69.{\displaystyle \underset{1}{\overset{3}{}}}\frac{e^x}{e^x-1}dx$

$70.{\displaystyle \underset{\frac{1}{4}}{\overset{5}{}}}\frac{e^{4x}1}{4x{e}^{4x}}dx$