Evaluate the integrals If possible:

a$){\displaystyle \underset{0}{\overset{}{}}}x{e}^{-x^2}dx$

b$){\displaystyle \underset{3}{\overset{}{}}}\frac{dx}{x+1}$

c$){\displaystyle \underset{-}{\overset{}{}}}\frac{dx}{x^2+16}$