阿肥剛剛在想微積分的事情，想說自己拿筆推推看。
第一個拿來開刀練習的是多項式的導數。
Q: Given f(x) = x^2, find its derivative.
A:
1. lim (f(x+h) - f(x)) / h, where h approches to 0
2. plug x^2 in the function, we have
((x + h)^2 - x^2) / h
which is equivalent to:
(x^2 + 2xh + h^2 - x^2) / h
=h(2x+h) / h
=2x+h
because h approches to 0, we plug 0 to h, which
gives us 2x
Therefore 2x is the derivative.