I will use  to stand for "numerator" and  to
stand for "denominator", so for instance,  stands for 
the first numerator,  stands for the first denominator,
 stands for the second numerator, etc.

Suppose your problem is of this type:



Get LCD of all denominators DEN1, DEN2, DEN3, and DEN4.

Form the fraction  and put it in parentheses


Multiply all four fractions by 



Now every one of those denominators will cancel 
entirely into the LCD, and the end result will be
much simpler, like this:



The smae principle works well regardless of how many fractions
you have on top or bottom.
--------------------------------------

1+2 over x divided by 7 over x



Write the  as  so that
everything will be a fraction, and put parentheses around
every fraction:



Get LCD of all denominators 1, x, and x.

Then LCD = x 

Form the fraction  and put it in parentheses


Multiply all three fractions by 



Now every one of those denominators will cancel 
entirely into the LCD, and the end result will be
much simpler, like this:



and that is simply:



-------------------------

a over b divided by a- 5a over b 



Write the  as  so everything will
be a fraction and put parentheses around everything:



Get LCD of all denominators , , and .

Form the fraction  and put it in parentheses


Multiply all four fractions by 



Now every denominator will cancel 
entirely into the LCD, and the end result will be
much simpler, like this:



which is simply



Now we can factor out  on the bottom:



Now the 's will cancel, leaving a  on top:

   


Answer:  

If I have time I'll come back and do the others.
But they're all done the same way.

Edwin