f(x) = a^x
lnf(x) = xlna
f'(x)/f(x) = lna
f'(x) = lna . f(x)
=lna . a^x
f'(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 -1 ) /h
=lim(h->0) a^x .( lna. h ) /h
=lna. a^x
追问下面的公式看懂了,上面都三行为什么f'(x)/f(x)等于lna呢?
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