Ответ:

  1. а) \[\frac{28x^4y^2}{21x^2y^3}=\frac{4x^2}{3y}.\]
  2. б) \[\frac{2x-6x^2}{6x-2}=\frac{2x(1-3x)}{2(3x-1)}=-x.\]
  3. в) \[\frac{4x^2-1}{1+4x+4x^2}=\frac{(2x-1)(2x+1)}{(2x+1)^2}=\frac{2x-1}{2x+1}.\]
  4. г) \[\frac{m^3-8n^3}{2n-m}=\frac{(m-2n)(m^2+2mn+4n^2)}{-(m-2n)}=-(m^2+2mn+4n^2).\]
  5. д) \[\frac{2^n x+2^{n+1}}{2^{n+1}x^3-2^{n+3}}=\frac{2^n(x+2)}{2^{n+1}(x^3-4)}=\frac{x+2}{2(x^3-4)}.\]

Ответ: а) \(\frac{4x^2}{3y}\); б) \(-x\); в) \(\frac{2x-1}{2x+1}\); г) \(-(m^2+2mn+4n^2)\); д) \(\frac{x+2}{2(x^3-4)}\).