28. Encontre $f'(a)$. $f(t) = 2t^3 +t$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac {f(t+h) - f(t)}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac { 2(t+h)^3 +t+h -(2t^3+t)}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac { 2(t^3+3t^2h+3th^2+h^3)+t+h-2t^3-t}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac { 2t^3+6t^2h+6th^2+2h^3+t+h-2t^3-t}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac { 6t^2h+6th^2+2h^3+h}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \frac { h(6t^2+6th+2h^2+1)}{h}$$

$$f'(x) = \lim\limits_{h \rightarrow 0} \ 6t^2+6th+2h^2+1  \ = 6t^2+6t \cdot 0+2 \cdot 0^2+1 \ = 6t^2+1$$