Find function `f(x)` satisfied `f(x)f(y)-f(xy)-90=10(x+y)/xy`

• This problem, I picked from Vietnamese book which related to finding function. It was a problem of Turkey National Olympiad First Round 2008 which also a difficult problem in Mathematics Calculation. 
• In Vietnamese language this problem:

• You can watch PDF solution and download it to be your document.

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