\[\boxed{\mathbf{4}\mathbf{.}}\]
\[f(x) = x^{2};\ \ x_{0} = 0;\ \ x_{2} = 2.\]
\[\textbf{а)}\ \mathrm{\Delta}x = 2 - 0 = 2.\]
\[\textbf{б)}\ \mathrm{\Delta}f = f\left( x_{2} \right) - f\left( x_{1} \right) =\]
\[= 4 - 0 = 4.\]
\[f(2) = 2^{2} = 4;\]
\[f(0) = 0^{2} = 0.\]
\[\textbf{в)}\ tg\ \beta = \frac{\mathrm{\Delta}f}{\mathrm{\Delta}x} = \frac{4}{2} = 2.\]