When I use MathJax, sometimes the fraction lines got a different thickness (below imgage). Why?
Another question is how to increase the vertical space between the lines, because in \begin{align} ... \end{align} the lines are so close to each other and it's hard to read them (as you can see on the below sample image).

\\[dimen]to add extra space between lines of an alignment. So\\[5pt]would add an extra 5 points of space between the lines at that breakpoint. – Davide Cervone Oct 10 '12 at 17:15\begin{align}\genfrac{}{}{1pt}{0}{a+b}{c+d}\frac{a+b}{c+d}– Lukas Nov 11 '15 at 21:43