Smath can be saved as 'sm' file and 'xmcd' file.
But why cannot open 'mcd' file?
What is the differences between 'sm' file and 'mcd' file?
Thank you!!!

Maybe you can convert mcd to xmcd,
there you have some posts on the forum about .mcd support :
http://en.smath.info/forum/yaf_postsm4377_open-mathcad-file.aspx#post4377
http://en.smath.info/forum/yaf_postsm5357_MathCAD.aspx#post5357

Wrote

Smath can be saved as 'sm' file and 'xmcd' file.
But why cannot open 'mcd' file?
What is the differences between 'sm' file and 'mcd' file?
Thank you!!!

*.sm and *.xmcd are both xml-based files (proprietary format open), *.mcd is proprietary format closed.

Thank you so much!!!