方法一:对数求导法
y = a^x
lny = ln(a^x) = x lna
1/y * dy/dx = lna * 1
dy/dx = lna * y
dy/dx = a^x lna
方法二:定义求导法
d/dx a^x = lim(h->0) [f(x + h) - f(x)]/h
= lim(h->0) [a^(x + h) - a^x]/h
= lim(h->0) (a^x a^h - a^x)/h
= a^x lim(h->0) (a^h - 1)/h
= a^x lim(h->0) [e^(h lna) - 1]/(h lna) * (lna)
= a^x lna lim(y->0) (e^y - 1)/y,令y = h lna
= a^x lna * 1
= a^x lna