Đáp án:
$1 - [\dfrac{1}{4} + \dfrac{1}{5} + \dfrac{1}{6} + \dfrac{1}{7}]$
$= 1 - [ \dfrac{1*210}{4*210} + \dfrac{1*168}{5*168} + \dfrac{1*140}{6*140} + \dfrac{1*120}{7*120}]$
$= 1 - \dfrac{638}{840}$
$= \dfrac{840}{840} - \dfrac{638}{840}$
$= \dfrac{202}{840}$
$= \dfrac{101}{420}$
@Yết gửi bạn nhé~~
HỌC TỐT~~
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