Đáp án:
Giải thích các bước giải:
1/
$\dfrac{(-5).7+5.(-4)}{2.2.5}$
$=\dfrac{5.(-7)+5.(-4)}{4.5}$
$=\dfrac{5.(-7-4)}{4.5}$
$=\dfrac{5.(-11)}{4.5}$
$=\dfrac{-11}{4}$
2/
$\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}$
$=\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}$
$=(\dfrac{2}{12}+\dfrac{1}{12})+(\dfrac{3}{60}+\dfrac{2}{60})$
$=\dfrac{1}{4}+\dfrac{1}{12}$
$=\dfrac{3}{12}+\dfrac{1}{12}$
$=\dfrac{1}{3}$
3/
$\dfrac{-8}{15}<\dfrac{x}{40}<\dfrac{-7}{15}$
$⇔\dfrac{(-8).8}{120}<\dfrac{3x}{120}<\dfrac{(-7).8}{120}$
$⇒(-64)<3x<(-56)$
⇒ 3x ∈ {-60; -57}
⇒ x ∈ {-20; -19}
Vậy x ∈ {-20; -19}
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