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  ## find the largest value of x
  
  find the largest value of x that satisfies:
  log2(x2)-log2(x+5)=2
  
  loga b  −  loga c  =  loga (b / c)
  loga x = b  ----->  ab = x
  ~~~~~~~~~~~
  
  log2 x2 − log2 (x + 5) = 2
  
  log2 [x2 / (x + 5)] = 2
  
  x2
  --------  =  22
  x + 5
  
  x2 = 4(x + 5)
  
  x2 - 4x - 20 = 0
  
  D = (-4)2 +80 = 96
  
  x12 = (4 ± √96) / 2
  
  x1 = 2 + 2√6 > 0
  
  x2 = 2 − 2√6 < 0 (logarithm is not exist for negative numbers and 0)
  
  x = 2(1 + √6)  is the answer.
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