Question 76828
My interpretation of your problem is that you need to simplify the rational
expression:
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{{{(r-13)/(13-r)}}}
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That being the case, notice what happens if you change the denominator by factoring 
out -1.  If you do that factoring you get:
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{{{(r-13)/((-1)*(-13+r))}}}
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and re-arranging the terms inside the parentheses in the denominator results in:
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{{{(r-13)/((-1)*(r-13))}}}
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Now you can cancel the like terms of the denominator and numerator to get:
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{{{cross(r-13)/((-1)*cross(r-13))}}}
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and this leaves you with just:
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{{{1/(-1)}}}
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So the result of this simplification is just plain {{{-1}}}
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Hope this helps you to see your way through the problem.
.