\(x=\frac{-1}{2}+\frac{3}{4}=\frac{-2}{4}+\frac{3}{4}=\frac{1}{4}\)

\(\frac{x}{5}=\frac{5}{6}-\frac{19}{30}=\frac{25}{30}-\frac{19}{30}=\frac{6}{30}=\frac{1}{5}\)

*\(1\div2\)không phải \(\frac{1}{2}\)à?*

\(\dfrac{x}{5}=\dfrac{5}{6}-\left(-\dfrac{19}{30}\right)\\ =>\dfrac{x}{5}=\dfrac{5}{6}+\dfrac{19}{30}\\ =>\dfrac{x}{5}=\dfrac{25}{30}+\dfrac{19}{30}\\ =>\dfrac{x}{5}=\dfrac{44}{30}\\ =>\dfrac{x}{5}=\dfrac{22}{15}\\ =>15x=22.5\\ =>15x=110\\ =>x=110:15\\ =>x=\dfrac{22}{3}\)

81

\(\dfrac{1}{2}+\dfrac{5}{6}+\dfrac{11}{12}+\dfrac{19}{20}+\dfrac{29}{30}+\dfrac{41}{42}+\dfrac{55}{56}+\dfrac{71}{72}+\dfrac{89}{90}=1-\dfrac{1}{2}+1-\dfrac{1}{6}+1-\dfrac{1}{12}+....+1-\dfrac{1}{90}=1+1+...+1-\left(\dfrac{1}{2}+\dfrac{1}{6}+...+\dfrac{1}{90}\right)=9-\left(\dfrac{1}{1x2}+\dfrac{1}{2x3}+...+\dfrac{1}{9x10}\right)=9-\left(\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+....+\dfrac{1}{9}-\dfrac{1}{10}\right)=9-\left(1-\dfrac{1}{10}\right)=9-\dfrac{9}{10}=\dfrac{81}{10}\)

\(\frac{5}{6}+\frac{\left(-19\right)}{30}=\frac{x}{5}\)

\(\frac{25}{30}+\frac{\left(-19\right)}{30}=\frac{x}{5}\)

\(\frac{6}{30}=\frac{x}{5}\)

\(\frac{1}{5}=\frac{x}{5}\)

\(\Rightarrow5x=5\times1\)

\(5x=5\)

\(x=5\div5\)

\(x=1\)

\(\frac{5}{6}+\frac{-19}{30}=\frac{x}{5}\)

\(\Leftrightarrow\frac{x}{5}=\frac{5}{6}+\frac{-19}{30}\)

\(\Leftrightarrow\frac{x}{5}=\frac{25}{30}+\frac{-19}{30}\)

\(\Leftrightarrow\frac{x}{5}=\frac{6}{30}\)

\(\Leftrightarrow\frac{x}{5}=\frac{1}{5}\)

\(\Rightarrow x=1\)

Vậy \(x=1\)

A = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72

   = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9

   = 1- 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9

   = 1 - 1/9 = 8/9

Câu B, C dấu * là nhân hay công vậy?

x/5    = 5/6 + -19/ 30

=> x/5      =      25/30 + -19/30

=> x/5      =    6/30

=> x/5      =    1/5

=>   x       =     1 ( vì mẫu số = 5 )

Vậy x = 1 

Chúc bạn học giỏi !!!

\(\frac{x}{5}=\frac{5}{6}+-\frac{19}{30}\)

\(\Rightarrow\frac{x}{5}=\frac{1}{5}\)

\(\Rightarrow x=1\)

học tốt ~~~~

\(\frac{x}{5}=\frac{5}{6}+\frac{-19}{30}\)

\(\Rightarrow\frac{x}{5}=\frac{25-19}{30}\)

\(\Rightarrow\frac{x}{5}=\frac{6}{30}\)

\(\Rightarrow\frac{x}{5}=\frac{1}{5}\)

\(\Rightarrow x=1\)

x = 5/6 + (-19)/30

x = 25/30 + (-19)/30

x = 6/30

x = 1/5 (t/m)

Kết luận:....

Chúc bạn học tốt, nhớ nha.

x = 5/6 + (-19)/30

   = 25/30 + (-19)/30

.  = 4/30

Nhớ k cho mình nha

Các bạn giúp mình với mình đang cần gấp lắm

\(\frac{7}{3}.9-\frac{9}{4}.5+\frac{11}{5}.6-\frac{13}{6}.7+\frac{15}{7}.8-\frac{17}{8}.9+\frac{19}{9}.10\)

\(=21-\frac{45}{4}+\frac{66}{5}-\frac{91}{6}+\frac{120}{7}-\frac{153}{8}+\frac{190}{9}\)

\(=26,91230159\)

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