Math

$$P\cdot Q = \|P\|\|Q\|\cos\alpha$$

$$\sum_{k=1}^5 a_k = a_1 + a_2 + a_3 + a_4 + a_5$$ $$\textstyle\sum_{k=1}^5$$ $$\textstyle\sum\limits_{k=1}^5$$ $$\sum\nolimits_{k=1}^5$$

$$y =\frac{1+x}{1-x}$$ $$y=(1+x)/(1-x)$$ $y=(1+x)/(1-x)$

I $\heartsuit$ you!

$$A = \left( \begin{array}{@{\,}cccc@{\,}} a_{11} & a_{12} & \ldots & a_{1n} \\ a_{21} & a_{22} & \ldots & a_{2n} \\ \vdots & \vdots & \ddots & \vdots \\ a_{m1} & a_{m2} & \ldots & a_{mn} \end{array} \right)$$