Đáp án:
Giải thích các bước giải:
$\dfrac{3}{35}+\dfrac{3}{63}+\dfrac{3}{99}+...+\dfrac{3}{x.(x-2)}=\dfrac{24}{35}$
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$⇒\dfrac{3}{2}.(\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}+...+\dfrac{2}{(x-2).x})=\dfrac{24}{35}$
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$⇒\dfrac{1}{5}-\dfrac{1}{x}=\dfrac{24}{35}.\dfrac{2}{3}$
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$⇒\dfrac{1}{5}-\dfrac{1}{x}=\dfrac{16}{35}$
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$⇒\dfrac{1}{x}=\dfrac{-9}{35}$
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$⇒\dfrac{-9}{-9x}=\dfrac{-9}{35}$
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$⇒-9x=35$
$⇒x=\dfrac{-35}{9}$
