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**Bacterius** Hi, I'm trying to derivate this function : $\displaystyle f(x) = \sqrt{2x - 1}$. I'm applying this principle : if $\displaystyle f = a^n$ then $\displaystyle f' = na^{n - 1}$. So there I go :

$\displaystyle f(x) = \sqrt{2x - 1}$

$\displaystyle f(x) = (2x - 1)^{\frac{1}{2}}$

Now I derivate :

$\displaystyle f'(x) = \frac{1}{2} (2x - 1)^{\frac{-1}{2}}$** <<<<<<< here! You forgot to apply the chain rule**

$\displaystyle f'(x) = \frac{1}{2} \frac{1}{(2x - 1)^{\frac{1}{2}}}$

$\displaystyle f'(x) = \frac{1}{2} \frac{1}{\sqrt{2x - 1}}$

$\displaystyle f'(x) = \frac{1}{2 \sqrt{2x - 1}}$

But the book says that's not right and the answer should be :

$\displaystyle f'(x) = \frac{2}{2 \sqrt{2x - 1}} = \frac{1}{\sqrt{2x - 1}}$

Does anyone see when I went wrong ? Thanks a lot ...