demo for reference of md or image for relative path

if you want to reference to another md, the path can be written in the format of relative path under the circumstance of file system. click here： demo1.md。

the same is to image.

demo for math typesetting（katex）

$E = mc^2$

$x = a_{1}^n + a_{2}^n + a_{3}^n$

$\sqrt[3]{X}$

$\sqrt{5 - x}$