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3/ Chu vi hình chữ nhật:
\(\left(\dfrac{1}{4}+\dfrac{3}{10}\right)\cdot2=\dfrac{11}{10}\) (chưa biết đơn vị)
Diện tích hình chữ nhật:
\(\dfrac{1}{4}\cdot\dfrac{3}{10}=\dfrac{11}{20}\) (chưa biết đơn vị)
1,=0 . [2017/2018+2018/2019]
=>0
2,TH1 x-3=0=>x=3
TH2 y-4=0=>y=4
3, -2/4 = -x/10 = 16/y
=>-1/2 = -x/10 = 16/y
=>-1/2 = -x/10 => -5/10 = -x/10 => x=5
-1/2 = 16/y => 16/-32 = 16/y => y = -32
a) \(\dfrac{1}{6},\dfrac{1}{3},\dfrac{1}{2},\dfrac{2}{3}\)
Vì \(\dfrac{1}{6}:\dfrac{1}{2}=\dfrac{1}{3}\) ; \(\dfrac{1}{3}:\dfrac{2}{3}=\dfrac{1}{2}\) nên suy ra \(\dfrac{1}{2}:\dfrac{3}{4}=\dfrac{2}{3}\)
Vậy số để điền vào chỗ trống là \(\dfrac{2}{3}\).
b) \(\dfrac{1}{8},\dfrac{5}{24},\dfrac{7}{24},\dfrac{3}{8}\)
Vì \(\dfrac{1}{8}:\dfrac{3}{5}=\dfrac{5}{24}\) ; \(\dfrac{5}{24}:\dfrac{5}{7}=\dfrac{7}{24}\) nên suy ra \(\dfrac{7}{24}:\dfrac{7}{9}=\dfrac{3}{8}\)
Vậy số để điền vào chỗ trống là \(\dfrac{3}{8}\).
c) \(\dfrac{1}{5},\dfrac{1}{4},\dfrac{3}{10},\dfrac{7}{20}\)
Vì \(\dfrac{1}{5}:\dfrac{4}{5}=\dfrac{1}{4}\) ; \(\dfrac{1}{4}:\dfrac{5}{6}=\dfrac{3}{10}\) nên suy ra \(\dfrac{3}{10}:\dfrac{6}{7}=\dfrac{7}{20}\)
Vậy số để điền vào chỗ trống là \(\dfrac{7}{20}\).
\(\left(1+\dfrac{7}{9}\right).\left(1+\dfrac{7}{20}\right).\left(1+\dfrac{7}{33}.\right)\left(1+\dfrac{7}{48}\right)...\left(1+\dfrac{7}{180}\right)\)
\(=\dfrac{16}{9}.\dfrac{27}{20}.\dfrac{40}{33}.\dfrac{55}{48}...\dfrac{7}{180}\)
\(=\dfrac{2.8}{1.9}.\dfrac{3.9}{2.10}.\dfrac{4.10}{3.11}.\dfrac{5.11}{4.12}...\dfrac{11.17}{10.18}\)
\(=\dfrac{\left(2.3.4.5...11\right).\left(8.9.10.11...17\right)}{\left(1.2.3.4...10\right).\left(9.10.11.12...18\right)}\)
\(=\dfrac{11.8}{1.18}=\dfrac{88}{18}=\dfrac{44}{9}\)
ta có ;
\(\left(1+\dfrac{7}{9}\right)\cdot\left(1+\dfrac{7}{20}\right).\left(1+\dfrac{7}{33}\right)...\left(1+\dfrac{1}{180}\right)\)
=\(\dfrac{16}{9}.\dfrac{27}{20}.\dfrac{40}{33}....\dfrac{187}{180}\)
=\(\dfrac{8.2}{9.1}.\dfrac{9.3}{10.2}.\dfrac{10.4}{3.11}.\dfrac{11.5}{4.12}....\dfrac{17.11}{18.10}\)
=\(\dfrac{8.9.10.11.12.13.14.15.16.17.2.3.4.5.6.7.8.9.10.11}{9.10.11.12.13.14.15.16.17.18.1.2.3.4.5.6.7.8.9.10}\)
=\(\dfrac{8.11}{18}=\dfrac{88}{18}=\dfrac{44}{9}\)
a: \(=\dfrac{6}{18}+\dfrac{3}{18}+\dfrac{2}{18}+\dfrac{1}{18}=\dfrac{15}{18}=\dfrac{5}{6}\)
b: \(=\dfrac{7}{4}-\dfrac{5}{4}-\dfrac{2}{4}=0\)
c: \(=\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+...+\dfrac{1}{9\cdot10}\)
\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\)
=1/2-1/10=4/10=2/5