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Thanks, I don't know how I can possibly thank you enough.

What's the difference between "\\" and "\par"?
Thanks

I have opened lots of documents, and want to close them except of the pointed one. How to do it? Thanks

Thanks

The numerator is too long and I want to put into several rows. How to do this?

For example

-{\frac {2\, \left( D_{{1,1}} \right) \left( f \right) \left( x+y,z+y,z+x \right) D_{{2}} \left( f \right) \left( x+y,z+y,z+x \right) \\
\mbox{}D_{{3}} \left( f \right) \left( x+y,z+y,z+x \right) -2\, \left ...