TeX is ignoring an optional value

Question

I have two tabular environments containing functions. The height of the boxes containing some of these functions is taller than the inter-line spacing in text mode. So, the appearance is cramped. I want to have \\ [1.2\baselineskip] separating the rows. This command is ignored. I tried using \\ [12\baselineskip] in one row. That is ignored, too. In the following code, only \\ [1cm] was not ignored. How do I get TeX to compile the code? (I want to keep the tabular environments.)

\documentclass[10pt]{amsart}

\begin{document}


\noindent {\textbf{Example}} \vskip1.25mm
The following functions are increasing on the interval $[0, \, \infty)$. \vskip1.25mm
\begin{tabular}{r @{\ }l}
{\textbf{i.)}}   &       $x^{2} + x$ \\[1.2\baselineskip]
{\textbf{ii.)}}  &       $x^{3} + 2x^{2} + 3x - 7$ \\[1.2\baselineskip]
{\textbf{iii.)}} &       $x^{2} + \sqrt{x}$
\end{tabular}
\vskip0.25in

\noindent {\textbf{Example}} \vskip1.25mm
The following functions are also increasing on the interval $[0, \, \infty)$. \vskip1.25mm
\begin{tabular}{r @{\ }l}
{\textbf{i.)}}   &       $\sqrt{x^{2} + x}$ \\[12\baselineskip]
{\textbf{ii.)}}  &       $\sqrt[\uproot{1} \leftroot{-1} 3]{x^{3} + 2x^{2} + 3x - 7}$ \\[1cm]
{\textbf{iii.)}} &       $\bigl(x^{2} + \sqrt{x}\bigr)^{3}$
\end{tabular}

\end{document}