Question 132993
Rationalize the denominator. Assume that all expressions under radicals represent positive numbers. 
(sqrt (c) - sqrt (d)) / (sqrt (c) + sqrt (d))
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Multiply numerator and denominator by (sqrt(c)-sqrt(d)) to get:
[sqrt(c)-sqrt(d)]^2/[c-d]

= [c+d -2sqrt(cd)]/(c-d)

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Cheers,
Stan H.