
<A NAME=MCD>

<TITLE>MCD</TITLE></A>
<b><a href=r37_idx.html>INDEX</a></b><p><p>



<B>MCD</B> _ _ _  _ _ _  _ _ _  _ _ _ <B>switch</B><P>
<P>
 
 <P>
<P>
When <em>mcd</em> is on, sums and differences of rational expressions are put 
on a common denominator. Default is <em>on</em>. 
<P>
<P>
 <P> <H3> 
examples: </H3>
<P><PRE><TT>
a/(x+1) + b/5; 

  5*A + B*X + B
  ------------- 
    5*(X + 1)


off mcd; 

a/(x+1) + b/5; 

         -1
  (X + 1)  *A + 1/5*B 


1/6 + 1/7; 

  13/42

</TT></PRE><P>Even with <em>mcd</em> off, rational expressions involving only nu
mbers are still 
put over a common denominator. 
<P>
<P>
Turning <em>mcd</em> off is useful when explicit negative powers are needed, 
or if no greatest common divisor calculations are desired, or when 
differentiating complicated rational expressions. Results when <em>mcd</em> 
is off are no longer in canonical form, and expressions equivalent to zero 
may not simplify to 0. Some operations, such as factoring cannot be done 
while <em>mcd</em> is off. This option should therefore be used with some 
caution. Turning <em>mcd</em> off is most valuable in intermediate parts of 
a complicated calculation, and should be turned back on for the last stage. 
<P>
<P>
<P>
