$u(x)=\dfrac{f(x)}{g(x)}$ $g(x)=\not 0$ ($g(x)\neq 0$)
$u(x+\Xi)=\dfrac{f(x+\Xi)}{g(x+\Xi)}$
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$u(x)$ deki değişim;
$\triangle u(x)=\dfrac{f(x+\Xi)}{g(x+\Xi)}-\dfrac{f(x)}{g(x)}$ düzenlersek
$\triangle u(x)=\dfrac{f(x+\Xi).g(x)-f(x).g(x+\Xi)}{g(x).g(x+\Xi)}$
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amacımız $\lim_{\Xi\rightarrow 0}\dfrac{\triangle u(x)}{\Xi}=u'(x)=\dfrac{f'(x).g(x)-f(x).g'(x)}{g(x)^2}$ olduğunu göstermek.
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Lafı fazla uzatmadan ;
$\lim_{\Xi \rightarrow 0}\dfrac{1}{g(x).g(x+\Xi)}=\dfrac{1}{g(x)^2}$ dir
$\lim_{\Xi \rightarrow 0}\dfrac{\triangle u(x)}{\Xi}=\left[\lim_{\Xi \rightarrow 0}\dfrac{f(x+\Xi)}{\Xi}.g(x)-\lim_{\Xi \rightarrow 0}\dfrac{g(x+\Xi)}{\Xi}.f(x) \right].\left(\dfrac{1}{g(x)^2}\right)$
$\lim_{\Xi \rightarrow 0}\dfrac{\triangle u(x)}{\Xi}=\dfrac{f'(x).g(x)-f(x).g'(x)}{g(x)^2}$
ispatlanır $\Box$