Question 1207360
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I'll go ahead and use your notation of cr(2) to represent the cube root of 2.<br>
The given expression has cr(2) in the denominator.  In the work you show, you multiplied numerator and denominator by cr(2).  But cr(2) times cr(2) does not rationalize the denominator.  You need to multiply numerator and denominator by cr(2) twice to make the denominator rational.<br>
(5/cr(2)*(cr(2)/cr(2))*(cr(2)/cr(2)) = (5*cr(2)*cr(2))/2 = 5cr(4)/2<br>