Đáp án:
Giải thích các bước giải:
`\frac{3x}{2.5}+\frac{3x}{5.8}+\frac{3x}{8.11}+\frac{3x}{11.14}-\frac{1}{21}`
$⇔x(\dfrac{3}{2.5}+\dfrac{3}{5.8}+...+\dfrac{3}{11.14})=\dfrac{1}{21}$
$⇔x(\dfrac{1}2-\dfrac{1}5+\dfrac{1}5-\dfrac{1}8+...+\dfrac{1}{11}-\dfrac{1}{14})=\dfrac{1}{21}$
$⇔x(\dfrac{1}2-\dfrac{1}{14})=\dfrac{1}{21}$
$⇔x×\dfrac{3}7=\dfrac{1}{21}$
$x=\dfrac{1}{21}:\dfrac{3}7=\dfrac{1}9$
Vậy `S={\frac{1}{9}}`
