Question 182831
Do you mean {{{3/(sqrt(7)-sqrt(2)) }}}???


If so, then the trick is to multiply numerator and denominator times the same thing as the denominator but with the opposite sign in the middle.  This is called the "conjugate" of the denominator.  In other words multiply numerator and denominator times {{{sqrt(7) + sqrt(2) }}}.  It looks like this:

{{{(3/(sqrt(7)-sqrt(2)))*((sqrt(7)+sqrt(2))/(sqrt(7)+sqrt(2) ) ) }}}


When you multiply out the denominator you get 
{{{3(sqrt(7) + sqrt(2) )/(7-2) }}}.
{{{3(sqrt(7) + sqrt(2) )/5 }}}.


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