SOLUTION: Five years later a boy's age will be 4 times the square of what it was 9 years ago. Find his present age.

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Question 871515: Five years later a boy's age will be 4 times the square of what it was 9 years ago. Find his present age.
Answer by Edwin McCravy(20056)   (Show Source): You can put this solution on YOUR website!
x       = the boy's age now
x+5     = the boy's age five years later
x-9     = what the boy's age was 9 years ago
(x-9)²  = the square of what it was 9 years ago  
4(x-9)² = 4 times the square of what it was 9 years ago


      x+5 = 4(x-9)²
      x+5 = 4(x-9)(x-9)
      x+5 = 4(x²-9x-9x+81)
      x+5 = 4(x²-18x+81)
      x+5 = 4x²-72x+324
        0 = 4x²-73x+319
        0 = (4x-29)(x-11)

     4x-29 =  0;    x-11 =  0
        4x = 29        x = 11  
         x =       

He couldn't have been  years old since 
9 years ago he wouldn't have even been born.  So

The boy's present age = 11

Checking:

Five years later the boy's age will be 16, which will be 
4 times the square of what it was 9 years ago.  9 years
ago, his age was 2.  The square of 2 is 4, and indeed 
16 is 4 times 4.

Edwin
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