Câu hỏi của cô gái lạnh lùng

Question

a,$\frac{2}{3}x-\frac{3}{2}\left(x-\frac{1}{2}\right)=\frac{5}{12}$b,$\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+........+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$

Trần Tiến Pro ✓ · Accepted Answer

$\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{1}{3.7}+\frac{1}{4.7}+\frac{1}{4.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{2.3.7}+\frac{2}{2.4.7}+\frac{2}{2.4.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{6.7}+\frac{2}{7.8}+\frac{2}{8.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{6}-\frac{2}{7}+\frac{2}{7}-\frac{2}{8}+....+\frac{2}{x}-\frac{2}{x+1}=\frac{2}{9}$$\Rightarrow\frac{2}{6}-\frac{2}{x+1}=\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{2}{6}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{1}{3}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{3}{9}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{1}{9}$$\Rightarrow\frac{2}{x+1}=\frac{2}{18}$$\Rightarrow x+1=18$$\Rightarrow x=17$Câu trả lời: $\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{1}{3.7}+\frac{1}{4.7}+\frac{1}{4.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{2.3.7}+\frac{2}{2.4.7}+\frac{2}{2.4.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{6.7}+\frac{2}{7.8}+\frac{2}{8.9}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{6}-\frac{2}{7}+\frac{2}{7}-\frac{2}{8}+....+\frac{2}{x}-\frac{2}{x+1}=\frac{2}{9}$$\Rightarrow\frac{2}{6}-\frac{2}{x+1}=\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{2}{6}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{1}{3}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{3}{9}-\frac{2}{9}$$\Rightarrow\frac{2}{x+1}=\frac{1}{9}$$\Rightarrow\frac{2}{x+1}=\frac{2}{18}$$\Rightarrow x+1=18$$\Rightarrow x=17$

zZz Cool Kid_new zZz · Answer

câu a khó quá.Để nghĩ.$\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{21\cdot2}+\frac{2}{28\cdot2}+\frac{2}{36\cdot2}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow2\left(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+....+\frac{1}{x\left(x+1\right)}\right)=\frac{2}{9}$$\Rightarrow\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+....+\frac{1}{x\left(x-1\right)}=\frac{1}{9}$$\Rightarrow\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+...+\frac{1}{x\left(x+1\right)}=\frac{1}{9}$$\Rightarrow\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{1}{9}$$\Rightarrow\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}$$\Rightarrow\frac{x-5}{6x+6}=\frac{1}{9}$$\Rightarrow9\left(x-5\right)=6x+6$$\Rightarrow9x-45=6x+6$$\Rightarrow9x-6x=51$$\Rightarrow3x=51$Tới đây bí:vCâu trả lời: câu a khó quá.Để nghĩ.$\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow\frac{2}{21\cdot2}+\frac{2}{28\cdot2}+\frac{2}{36\cdot2}+.....+\frac{2}{x\left(x+1\right)}=\frac{2}{9}$$\Rightarrow2\left(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+....+\frac{1}{x\left(x+1\right)}\right)=\frac{2}{9}$$\Rightarrow\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+....+\frac{1}{x\left(x-1\right)}=\frac{1}{9}$$\Rightarrow\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+...+\frac{1}{x\left(x+1\right)}=\frac{1}{9}$$\Rightarrow\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{1}{9}$$\Rightarrow\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}$$\Rightarrow\frac{x-5}{6x+6}=\frac{1}{9}$$\Rightarrow9\left(x-5\right)=6x+6$$\Rightarrow9x-45=6x+6$$\Rightarrow9x-6x=51$$\Rightarrow3x=51$Tới đây bí:v

Trần Tiến Pro ✓ · Answer

$\frac{2}{3}x-\frac{3}{2}\left(x-\frac{1}{2}\right)=\frac{5}{12}$$\Rightarrow\frac{2}{3}x-\frac{3}{2}x+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\left(\frac{2}{3}x-\frac{3}{2}x\right)+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\frac{-5}{6}x+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\frac{-5}{6}x=\frac{5}{12}-\frac{3}{4}$$\Rightarrow\frac{-5}{6}x=\frac{5}{12}-\frac{9}{12}$$\Rightarrow\frac{-5}{6}x=\frac{-4}{12}$$\Rightarrow\frac{-5}{6}x=\frac{-1}{3}$$\Rightarrow x=\frac{-1}{3}:\frac{-5}{6}$$\Rightarrow x=\frac{2}{5}$Câu trả lời: $\frac{2}{3}x-\frac{3}{2}\left(x-\frac{1}{2}\right)=\frac{5}{12}$$\Rightarrow\frac{2}{3}x-\frac{3}{2}x+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\left(\frac{2}{3}x-\frac{3}{2}x\right)+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\frac{-5}{6}x+\frac{3}{4}=\frac{5}{12}$$\Rightarrow\frac{-5}{6}x=\frac{5}{12}-\frac{3}{4}$$\Rightarrow\frac{-5}{6}x=\frac{5}{12}-\frac{9}{12}$$\Rightarrow\frac{-5}{6}x=\frac{-4}{12}$$\Rightarrow\frac{-5}{6}x=\frac{-1}{3}$$\Rightarrow x=\frac{-1}{3}:\frac{-5}{6}$$\Rightarrow x=\frac{2}{5}$

Khoá học trên OLM (olm.vn)

Khoá học trên OLM (olm.vn)