\[\boxed{\mathbf{26}\mathbf{.}}\]
\[\textbf{а)}\ y = 3x + 5\]
\[y^{'} = 3x^{'} + 5^{'} = 3;\]
\[dy = 3\mathrm{\Delta}x.\]
\[\textbf{б)}\ y = x^{2} + 2x + 4\]
\[y^{'} = \left( x^{2} \right)^{'} + 2x^{'} + 4' = 2x + 2;\]
\[dy = (2x + 2)\mathrm{\Delta}x.\]
\[\textbf{в)}\ y = x^{3} - 5x + 11\]
\[y^{'} = \left( x^{3} \right)^{'} - 5x^{'} + 11^{'} = 3x^{2} - 5;\]
\[dy = (3x^{2} - 5)\mathrm{\Delta}x.\]