Chapter 3.6, Problem 57E

You just got your derivatives test back, and you can’t under- stand why that teacher of yours deducted so many points for what you thought was your best work:

$\begin{array}{l}f\text{'}\left(a\right)=\underset{h\to 0}{\lim }\frac{f\left(x+h\right)-f\left(x\right)}{h}\\ =\underset{h\to 0}{\lim }\frac{f\left(x\right)+h-f\left(x\right)}{h}\end{array}$

$=\underset{h\to 0}{\lim }\frac{h}{h}$

Canceled the $f\left(x\right)$

$=1.$

$\text{X WRONG }-10$

What was wrong with your answer? (There may be more than one error.)