About black triangles, if someone wants to use $\displaystyle \blacktriangleright$ or $\displaystyle \blacktriangleleft$ just click on the image :D

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Now watch

$\displaystyle \frac{2}{6} = \frac{2}{{2 \cdot 3}} = \frac{{\mathop {\rlap/2}\limits^1 }}{{\mathop {\rlap/2}\limits_1 \cdot 3}} = \frac{1}{3}.$

Really cool, isn't it? I made it with MathType. To generate the cross line, you gotta put {\rlap/}. See the code above.