\usepackage{hyperref}
\begin{equation}
\href{http://www.wolframalpha.com/input/?i=integral+of+\%281+-+x\%5E2\%29\%5E.5+dx+on+-1+to+1}{
\int_{-1}^1 \sqrt{1 - x^2} dx = \frac{1}{2} \pi
}
\end{equation}