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a)=1/2*2/3......*19/20
=1/20
b)=3/2*4/3......*2008/2007
=3/2007
\(x\)là dấu nhân hả bạn? Nếu vậy thì mk làm cho nhé
\(A=\left(1-\frac{1}{2}\right)\cdot\left(1-\frac{1}{3}\right)\cdot\left(1-\frac{1}{4}\right)\cdot....\cdot\left(1-\frac{1}{20}\right)\)
\(A=\frac{1}{2}\cdot\frac{2}{3}\cdot\frac{3}{4}\cdot.......\cdot\frac{17}{18}\cdot\frac{18}{19}\cdot\frac{19}{20}=\frac{1}{20}\)
Vậy \(A=\frac{1}{20}\)
\(B=1\frac{1}{2}\cdot1\frac{1}{3}\cdot1\frac{1}{4}\cdot........\cdot1\frac{1}{2005}\cdot1\frac{1}{2006}\cdot1\frac{1}{2007}\)
\(B=\frac{3}{2}\cdot\frac{4}{3}\cdot\frac{5}{4}\cdot......\cdot\frac{2006}{2005}\cdot\frac{2007}{2006}\cdot\frac{2008}{2007}=\frac{2008}{2}=1004\)
Vậy \(B=1004\)
DẤU CHẤM LÀ DẤU NHÂN
a,
\(=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}....\frac{19}{20}=\frac{1}{20}\)
b, \(1\frac{1}{2}.1\frac{1}{3}....1\frac{1}{2017}=\frac{3}{2}.\frac{4}{3}....\frac{2018}{2017}=\frac{2018}{2}=1009\)
1-1/3=2/3; 1-1/4=3/4; 1-1/5=4/5....; 1-1/99=98/99
=> A= (2.3.4.5....98):(3.4.5....99)=2/99
Đs: 2/99
\(a,x-\frac{5}{6}:1\frac{1}{6}=0,125\)
\(x-\frac{5}{6}:\frac{7}{6}=\frac{1}{8}\)
\(x-\frac{5}{7}=\frac{1}{8}\)
\(x=\frac{1}{8}+\frac{5}{7}\) \(x=\frac{47}{56}\)
\(b,\left(1-\frac{2}{10}+x+\frac{1}{5}\right):\left(1\frac{1}{3}-\frac{2}{3}+3\frac{1}{3}\right)-1=1\frac{1}{2}\)
\(\left(1-\frac{1}{5}+x+\frac{1}{5}\right):\left(\frac{4}{3}-\frac{2}{3}+\frac{10}{3}\right)-1=\frac{3}{2}\)
\(\left(\frac{4}{5}+x+\frac{1}{5}\right):4=\frac{3}{2}+1\)
\(\left(1+x\right):4=\frac{5}{2}\)
\(1+x=\frac{5}{2}.4\)
\(1+x=10\)
\(x=10-1\)
\(x=9\)
BÀI 1:
\(541-\left(125-x\right)\times2\frac{2}{3}=469\)
\(\Rightarrow541-\left(125-x\right)\times\frac{8}{3}=469\)
\(\Rightarrow541-\frac{1000}{3}+\frac{8}{3}\times x=469\)
\(\Rightarrow\frac{623}{3}-\frac{8}{3}\times x=469\)
\(\Rightarrow\frac{8}{3}\times x=\frac{784}{3}\)
\(\Rightarrow x=98\)
BÀI 2 :
\(A=\frac{1}{2\times3}+\frac{1}{2\times3\times4}+\frac{1}{3\times4\times5}+...\)\(+\frac{1}{18\times19\times20}\)
\(=\frac{1}{2}\times\left(\frac{1}{1\times2}+\frac{1}{2\times3}\right)+\frac{1}{2}\times\left(\frac{1}{2\times3}+\frac{1}{3\times4}\right)\)\(+...+\frac{1}{2}\times\left(\frac{1}{18\times19}+\frac{1}{19\times20}\right)\)
\(=\frac{1}{2}\times\left(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{2\times3}+...+\frac{1}{19\times20}\right)\)
\(=\frac{1}{2}\times\left(\frac{1}{1\times2}-\frac{1}{19\times20}\right)\)
\(=\frac{1}{2}\times\frac{189}{380}\)
\(=\frac{189}{760}\)
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