Question : If $\sqrt{\frac{a}{b}}=\frac{8}{3}+\sqrt{\frac{b}{a}}$ and $(a+b)=30$, then what is the value of $ab$?
Option 1: 64
Option 2: 28
Option 3: 81
Option 4: 36
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Correct Answer: 81
Solution :
We have:
$\sqrt{\frac{a}{b}} - \sqrt{\frac{b}{a}} = \frac{8}{3}$
Squaring both sides,
$\frac{a}{b} + \frac{b}{a} - 2 = \frac{64}{9}$
Or, $\frac{a^2 + b^2}{ab} = \frac{64}{9} + 2$
Or, $\frac{a^2 + b^2}{ab} = \frac{82}{9}$...........................(i)
From the given condition,
$a + b = 30$
Or, $(a + b)^2 = 30^2 $
Or, $a^2 + b^2 + 2ab = 900$
Or, $a^2 + b^2 =900-2ab$.......................................(ii)
Substituting equation (ii) into equation (i),
$\frac{900 - 2ab}{ab} = \frac{82}{9}$
Or, $9(900 - 2ab)=82ab$
Or, $100ab=8100$
Or, $ab = 81$
Hence, the correct answer is 81.
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