(1) $\int xe^{-x^{2}}dx= \underline { \quad \quad \quad }$.

(2) $\int \dfrac {x}{1+x^{4}}dx= \underline { \quad \quad \quad }$.

(3) $\int \dfrac {2x+1}{ \sqrt {x^{2}+x}}dx= \underline { \quad \quad \quad }$.

(4) $\int x \sqrt {x^{2}+4}dx= \underline { \quad \quad \quad }$.