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a;
c; \(\dfrac{1}{10}\) + \(\dfrac{4}{20}\) + \(\dfrac{9}{30}\)+\(\dfrac{16}{40}+\dfrac{25}{50}+\dfrac{36}{60}+\dfrac{49}{70}+\dfrac{64}{80}+\dfrac{81}{90}\)
= \(\dfrac{1}{10}+\dfrac{2}{10}+\dfrac{3}{10}+\dfrac{4}{10}+\dfrac{5}{10}+\dfrac{6}{10}+\dfrac{7}{10}+\dfrac{8}{10}+\dfrac{9}{10}\)
= \(\dfrac{1+2+3+4+5+6+7+8+9}{10}\)
= \(\dfrac{\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5}{10}\)
= \(\dfrac{10+10+10+10+5}{10}\)
= \(\dfrac{\left(10+10+10+10\right)+5}{10}\)
= \(\dfrac{10\times4+5}{10}\)
= \(\dfrac{45}{10}\)
= \(\dfrac{9}{2}\)
\(\frac{2}{3}\times\frac{3}{4}\times\frac{4}{5}=\frac{2}{5}\)
\(\frac{1}{2}:\frac{3}{4}:\frac{5}{6}=\frac{1}{2}\times\frac{4}{3}\times\frac{6}{5}=\frac{1\times2\times2}{5}=\frac{4}{5}\)
Viết kết quả phép chia dưới dạng phân số và số thập phân:
a) \(3:4\) b) \(7:5\) c) \(1:2\) d) \(7:4\)
a) 3 : 4 = 3/4 = 0,75.
b) 7 : 5 = 7/5 = 1,4 .
c) 1 : 2 = 1/2 = 0,5
d) 7 : 4 = 7/4 = 1,75.
\(\dfrac{4}{5}+x=3\dfrac{1}{5}\)
\(\Leftrightarrow\dfrac{4}{5}+x=\dfrac{16}{5}\)
\(\Leftrightarrow0,8+x=3,2\)
\(\Leftrightarrow x=3,2-0,8\)
\(\Leftrightarrow x=2,4\)
\(\dfrac{4}{5}+x=3\dfrac{1}{5}\)
⇔ \(\dfrac{4}{5}+x=\dfrac{16}{5}\)
⇔ \(x=\dfrac{12}{5}=2,4\)
\(\frac{1}{2}=0,5\);\(\frac{1}{5}=0,2\)
Ta có:\(0,5:x=0,2\)
=> \(x=0,5:0,2\)
=> \(x=2,5\)
Vậy \(x=2,5\)
\(\frac{1}{2}:x=\frac{1}{5}\)
\(\Rightarrow x=\frac{1}{2}:\frac{1}{5}\)
\(\Rightarrow x=\frac{1}{2}\cdot\frac{5}{1}\)
\(\Rightarrow x=\frac{5}{2}\)
\(\Rightarrow x=2,5\)
\(\frac{8}{9}\):\(\frac{3}{4}\).\(\frac{5}{8}\)
=\(\frac{8}{9}\).\(\frac{4}{3}\).\(\frac{5}{8}\)
=\(\frac{8.4.5}{9.3.8}\)
=\(\frac{160}{216}\)
=0,(740)
4.a. 1/2 + 1/4 + 1/5
= 1*10/20 + 1*5/20 + 1*4/20
= (10 + 5 + 4)/20
= 19/20
= 0,95
b.1/2 * 4/5 : 1/4
= (1*4/2*5) : 1/4
= 4/10 : 1/4
= 2/5 * 4
= 8/5
= 1,6
`a) 1/2 + 1/4 + 1/5`
`=10/20 + 5/20 + 4/20`
`= 19/20`
`b)1/2xx 4/5 : 1/4`
`= 4/10 : 1/4`
`= 8/5`