Question 85111
c - d
------
1 - 1
-   -
d   c
---------------

or (c-d) / [1/d - 1/c]
Simplify the denominator to get:

= (c-d) / [(c-d)/cd]
Invert the denominator and multiply

= (c-d)*[cd/(c-d)]
Cancel the common (c-d) to get:

= cd
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Cheers,
Stan H.