(1)

\begin{align*} erfc(x) & =1-erf(x)\\ & =\frac{2}{\sqrt{\pi}}\int_{0}^{\infty}e^{-t^{2}}dt-\frac{2}{\sqrt{\pi}}\int_{0}^{x}e^{-t^{2}}dt\\ & =\frac{2}{\sqrt{\pi}}\int_{x}^{\infty}e^{-t^{2}}dt \end{align*}

(2)

\begin{align*} erfi(x) & =-ierf(ix)\\ & =-i\frac{2}{\sqrt{\pi}}\int_{0}^{ix}e^{-t^{2}}dt\\ & =\frac{2}{\sqrt{\pi}}\int_{0}^{x}e^{s^{2}}ds\cmt{s=-it} \end{align*}