This is a test :)
hidden:
\[ \frac{|ax + by + c|}{\sqrt{a^{2}+b^{2}}} \] \[ |a|<1 \]
\[\begin{equation}\label{eq1} e=mc^2 \end{equation}\]
\[ 1=\sum_{i=0}^{m}\sum_{k=0}^{W_i-1}b_{i,k}=\sum_{i=0}^{m}b_{i,0}\sum_{k=0}^{W_i-1}\frac{W_i-k}{W_i}=\sum_{i=0}^{m}b_{i,0}\frac{W_i+1}{2}\\ =\frac{b_{0,0}}{2}\left[W\left(\sum_{i=0}^{m-1}(2p)^i+\frac{(2p)^m}{1-p}\right) + \frac{1}{1-p}\right] \]
\[ \cos (2\theta) = \cos^2 \theta - \sin^2 \theta \] \[ \forall x \in X, \quad \exists y \leq \epsilon \]