giúp tớ baì giai thừa với

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$\dfrac{1}{2!}+\dfrac{2}{3!}+...+\dfrac{99}{100!}$

$=\dfrac{2-1}{2!}+\dfrac{3-1}{3!}+...+\dfrac{100-1}{100!}$

$=\dfrac{1}{1!}-\dfrac{1}{2!}+\dfrac{1}{2!}-\dfrac{1}{3!}+...+\dfrac{1}
{99!}-\dfrac{1}{100!}=1-\dfrac{1}{100!}$


 
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