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by srcc25anu » Tue May 14, 2013 12:57 pm
√80 can be written as 4√5. So 3√80 = 3*4√5 or 12√5

Multiply right side of the expression (both numerator and denominator) by (9-4√5) that will give:
=> [3 * (9-4√5)] / [(9+4√5) * (9-4√5)]
=>(27 - 12√5) / (81-80)
=> (27- 12√5)

Adding left side of the expression to the right side we get: root(12√5 + 27- 12√5)
=√27 = 3√3
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