(2√7)/(√3-√5)
The trick here is just to multiply by the conjugate of the denominator in both the numerator and denominator....so we have.....
[(2√7)/(√3-√5)] *[(√3+√5) / (√3+√5)] =
[2√(21) + 2√(35)] / (3-5) =
[(2)[√(21) + √(35)]] / (-2) =
-[√(21) + √(35)]
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The next one is similar........
(4x)/(3-√6) * [ (3+√6) / (3+√6)] =
[(4x)(3+√6)]/[9 -6] =
[12x + 4x√6] / [3]