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Identify any non-permissible values, and simplify the expressions.

1. \(\frac{{9d}}{{4f^2}} - \frac{{3d}}{{4f^2 }}\)
   
   
   
2. \(\frac{6}{{c\left( {c + 2} \right)}} - \frac{2}{c}\)
   
   
   
3. \(\frac{{8x}}{{4x + 1}} + \frac{3}{{4x^2 + 13x + 3}}\)