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Bài 2 :
a, \(3+\dfrac{2}{5}\)
= \(\dfrac{15}{5}+\dfrac{2}{5}\)
= \(\dfrac{17}{5}\)
b, \(4-\dfrac{5}{7}\)
= \(\dfrac{28}{7}-\dfrac{5}{7}\)
= \(\dfrac{23}{7}\)
c, \(1-\left[\dfrac{2}{5}+\dfrac{1}{3}\right]\)
= \(1-\left[\dfrac{6}{15}+\dfrac{5}{15}\right]\)
= \(\dfrac{15}{15}-\dfrac{11}{15}\)
= \(\dfrac{4}{15}\)
Bài 2
a, 3 + \(\dfrac{2}{5}\) = \(\dfrac{15}{5}\) + \(\dfrac{2}{5}\) = \(\dfrac{17}{5}\)
b, 4 - \(\dfrac{5}{7}\) = \(\dfrac{28}{7}\) - \(\dfrac{5}{7}\) = \(\dfrac{23}{7}\)
c, 1 - ( \(\dfrac{2}{5}\) + \(\dfrac{1}{3}\))
= 1 - (\(\dfrac{6}{15}\) + \(\dfrac{5}{15}\))
= 1 - \(\dfrac{11}{15}\)
= \(\dfrac{15}{15}\) - \(\dfrac{11}{15}\)
= \(\dfrac{4}{15}\)
b, \(\dfrac{3}{5}\) - \(\dfrac{3}{8}\)
= \(\dfrac{24}{40}\) - \(\dfrac{15}{40}\)
= \(\dfrac{9}{40}\)
c, \(\dfrac{1}{4}\) + \(\dfrac{5}{6}\)
= \(\dfrac{6}{24}\) + \(\dfrac{20}{24}\)
= \(\dfrac{26}{24}\)
= \(\dfrac{13}{12}\)
100km gấp 50 km số lần là:
100:50=2(lần)
Ô tô đó đi quãng đường 50km thì tiêu thụ:
12:2=6(l xăng)
6/25= 24/100 (nhân cho 4)
500/1000 = 50/100( chia cho 10)
18/200= 9/100 (chia cho 2)
b)\(4\times\dfrac{3}{8}=\dfrac{12}{8}=\dfrac{3}{2}\)
\(3:\dfrac{1}{2}\)
=\(3\times\dfrac{2}{1}\)
= 6
=\(\dfrac{1}{2}:3\)
= \(\dfrac{1}{2}\times\dfrac{1}{3}\)
= \(\dfrac{1}{6}\)
1a, \(\dfrac{3}{10}\) \(\times\) \(\dfrac{4}{9}\)
= \(\dfrac{3\times2\times2}{2\times5\times3\times3}\)
= \(\dfrac{2}{15}\)
\(\dfrac{6}{5}\): \(\dfrac{3}{7}\)
= \(\dfrac{6}{5}\) \(\times\) \(\dfrac{7}{3}\)
= \(\dfrac{14}{5}\)
\(\dfrac{3}{4}\) \(\times\) \(\dfrac{2}{5}\)
= \(\dfrac{3\times2}{2\times2\times5}\)
= \(\dfrac{3}{10}\)
\(\dfrac{5}{8}\) : \(\dfrac{1}{2}\)
= \(\dfrac{5}{8}\) \(\times\) \(\dfrac{2}{1}\)
= \(\dfrac{5}{4}\)