Evaluating a function

If f(x) = x^(-2/3), what is the value of f(8) ?

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f(x) = x^(-2/3)

      1
--------- = f(x)   {made a positive exponent by moving into denominator}
  x^(2/3)

      1
--------- = f(8)   {substituted 8 for x}
  8^(2/3)

       1
---------- = f(8)   {8 to the two-thirds power is the same as the cube root of 8, squared}
    (³√8)²

  1
----- = f(8)   {evaluated the cube root of 8 to be 2}
  2²

 1
--- = f(8)   {squared the 2}
 4

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