\(x\times\dfrac{20}{11}=\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}\\ x\times\dfrac{20}{11}=\dfrac{10}{11}\\ \Rightarrow x=\dfrac{1}{2}\)
Ta có :
=> X x 20/11 = ( 1 - 1/3 ) + ( 1/3 - 1/5 ) + ( 1/5 - 1/7 ) + ( 1/7 - 1/9 ) + ( 1/9 + 1/ 11 )
=> X x 2/11 = 1 - 1/11 = X x 1/11 = 11/11 - 1/11
=> X x 20/11 = 10/11
=> X = 10/11 : 20/11
=> X = 1/2
\(Xx\dfrac{20}{11}=\dfrac{2}{1}-\dfrac{2}{3}+\dfrac{2}{3}-\dfrac{2}{5}+\dfrac{2}{5}-\dfrac{2}{7}+\dfrac{2}{7}-\dfrac{2}{9}+\dfrac{2}{9}-\dfrac{2}{11}\)
\(Xx\dfrac{20}{11}=\dfrac{2}{1}-\dfrac{2}{11}\)
\(Xx\dfrac{20}{11}=\dfrac{20}{11}\)
\(X=\dfrac{20}{11}:\dfrac{20}{11}\)
\(X=1\)
vậy X = 1