\[\boxed{\mathbf{31}\mathbf{.}}\]
\[\textbf{а)}\ y = x^{4}\]
\[y^{'} = \left( x^{3} \cdot x \right)^{'} = \left( x^{3} \right)^{'}x +\]
\[+ x^{3} \cdot x^{'} = 3x^{2} \cdot x + x^{3} \cdot 1 = 4x^{3}.\]
\[\textbf{б)}\ y = x^{5}\]
\[y^{'} = \left( x^{4} \cdot x \right)^{'} = \left( x^{4} \right)^{'} \cdot x +\]
\[+ x^{4} \cdot x^{'} = 4x^{3} \cdot x + x^{4} \cdot 1 = 5x^{4}.\]
\[\textbf{в)}\ y = x^{6}\]
\[y^{'} = \left( x^{5} \cdot x \right)^{'} = \left( x^{5} \right)^{'} \cdot x +\]
\[+ x^{5} \cdot x^{'} = 5x^{4} \cdot x + x^{5} \cdot 1 = 6x^{5}.\]
\[\textbf{г)}\ y = x^{7}\]
\[y^{'} = \left( x^{6} \cdot x \right)^{'} = \left( x^{6} \right)^{'} \cdot x +\]
\[+ x^{6} \cdot x^{'} = 6x^{5} \cdot x + x^{6} \cdot 1 = 7x^{6}.\]