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cách phòng tránh

\(s=\)\(\dfrac{1}{1\cdot3}+\dfrac{1}{3\cdot5}+...+\dfrac{1}{9\cdot11}\)

=\(\dfrac{1}{2}\cdot\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+..+\dfrac{1}{9}-\dfrac{1}{11}\right)\)

=\(\dfrac{1}{2}\cdot\left(1-\dfrac{1}{11}\right)\)

=\(\dfrac{1}{2}\cdot\dfrac{10}{11}\)

=\(\dfrac{5}{11}\)

12

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\(\dfrac{\dfrac{2}{3}-\dfrac{2}{5}+\dfrac{2}{9}+\dfrac{2}{13}}{\dfrac{11}{3}-\dfrac{11}{5}+\dfrac{11}{9}+\dfrac{11}{13}}\)

\(=\dfrac{2\cdot\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{9}+\dfrac{1}{13}\right)}{11\cdot\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{9}+\dfrac{1}{13}\right)}\)

\(=\dfrac{2}{11}\)

\(\left(3\dfrac{1}{4}-6\dfrac{5}{9}\right)\cdot2\dfrac{2}{27}:\left(0,5-\dfrac{3}{5}\right)\)

=\(\left(\dfrac{13}{4}-\dfrac{59}{9}\right)\cdot\dfrac{56}{27}:\left(\dfrac{1}{2}-\dfrac{3}{5}\right)\)

= \(-\dfrac{119}{36}\cdot\dfrac{56}{27}:\left(-\dfrac{1}{10}\right)\)\

=\(-\dfrac{1666}{243}:\left(-\dfrac{1}{10}\right)\)

=\(\dfrac{1660}{243}\)

vì x+y+z = 1

và \(x^3+y^3+z^3=1\)

\(\Rightarrow\)P=1

1.x=4

2.x=6

3.x=8