26:
\(\dfrac{4}{x^2-1}=\dfrac{4}{\left(x-1\right)\left(x+1\right)}\)
\(\dfrac{2x+1}{x+1}=\dfrac{\left(2x+1\right)\left(x-1\right)}{\left(x+1\right)\left(x-1\right)}\)
27: \(\dfrac{3x}{x^2+2x+1}=\dfrac{6x}{2\left(x+1\right)^2}\)
\(\dfrac{3}{2x+2}=\dfrac{3\left(x+1\right)}{2\left(x+1\right)^2}\)
28:
\(\dfrac{2}{x^3-8}=\dfrac{2}{\left(x-2\right)\left(x^2+2x+4\right)}\)
\(\dfrac{x}{x^2+2x+4}=\dfrac{x\left(x-2\right)}{\left(x-2\right)\left(x^2+2x+4\right)}\)