$\frac{da^x}{dx}=0$?

Question

I know that $\frac{da^x}{dx}=a^x ln(a)$, but I tried to differentiate it in another way and I found it strange.

$$f(x)=\overbrace{0.5*0.5*0.5...}^x$$ $$ln[f(x)]=\sum_{i=1}^{x} ln(0.5)$$ $$\frac{d}{dx}ln[f(x)]=\frac{d}{dx}\sum_{i=1}^{x} ln(0.5)$$ $$\frac{1}{f(x)}\frac{df(x)}{dx}=\sum_{i=1}^{x}\frac{d}{dx} ln(0.5)$$ $$\frac{1}{f(x)}\frac{df(x)}{dx}=\sum_{i=1}^{x}0$$ $$\frac{df(x)}{dx}=0$$

Why I get a zero?

EDIT: How to do derivative with respect to the upper limit of the sum?

Answer 1

Just differentiate the upper limit: $$\frac{1}{f(x)} \frac{df(x)}{dx} = \sum_{i=1}^{dx/dx} \ln(0.5) = \ln (0.5)$$ so that $$\frac{df(x)}{dx} = f(x) \ln (0.5) = (0.5)^x \ln (0.5).$$

Answer 2

$$\frac{1}{f(x)}\frac{df(x)}{dx}=\sum_{i=1}^{x}\frac{d}{dx} ln(0.5)$$

Can't do that, you can't put the derivative inside the sum when the upper limit of the sum is a function of $x$