(a) f(x) = x^3 + x
f´(x₀) \( =\lim \limits_{h \rightarrow 0} \frac{\left(x_{0}+h\right)^{3}+\left(x_{0}+h\right)-x_{0}^{3}-x_{0}}{h}= \)
\( =\lim \limits_{h \rightarrow 0} \frac{x_{0}^{3}+3 x_{0}^{2} \cdot h+3 x_{0} \cdot h^{2}+h^{3}+x_{0}+h-x_{0}^{3}-x_{0}}{h}= \)
\( =\lim \limits_{h \rightarrow 0} \frac{3 x_{0}^{2} \cdot h+3 x_{0} \cdot h^{2}+h^{3}+h}{h}= \)
\( =\lim \limits_{h \rightarrow 0}\left(3 x_{0}^{2}+3 x_{0} \cdot h+h^{2}+1\right)= \)
\( =3 x_{0}^{2}+1 \)