can gi phai biet cach day 

a,\(\frac{x+1}{10}+\frac{x+1}{11}+\frac{x+1}{12}=\frac{x+1}{13}+\frac{x+1}{14}\)

= \(\frac{x+1}{10}+\frac{x+1}{11}+\frac{x+1}{12}-\frac{x+1}{13}-\frac{x+1}{14}\)

\(\Rightarrow\left(x+1\right)\left(\frac{1}{10}+\frac{1}{11}+\frac{1}{12}-\frac{1}{13}-\frac{1}{14}\right)\)

Vì 10<11<12<13<14 \(\Rightarrow\frac{1}{10}>\frac{1}{11}>\frac{1}{12}>\frac{1}{13}>\frac{1}{14}\)

\(\Rightarrow\frac{1}{10}+\frac{1}{11}+\frac{1}{12}-\frac{1}{13}-\frac{1}{14}>0\)

\(\Rightarrow x+1=0\)

\(\Rightarrow x=-1\)

b, \(\frac{x+4}{2000}+\frac{x+3}{2001}=\frac{x+2}{2002}+\frac{x+1}{2003}\)

\(=\left(\frac{x+4}{2000}+1\right)+\left(\frac{x+3}{2001}+1\right)=\left(\frac{x+2}{2002}+1\right)\)\(+\left(\frac{x+1}{2003}+1\right)\)

\(=\frac{x+2004}{2000}+\frac{x+2004}{2001}=\frac{x+2004}{2002}+\frac{x+2004}{2003}\)

\(=\frac{x+2004}{2000}+\frac{x+2004}{2001}-\frac{x+2004}{2002}-\frac{x+2004}{2003}=0\)

\(=\left(x+2004\right)\left(\frac{1}{2000}+\frac{1}{2001}-\frac{1}{2002}-\frac{1}{2003}\right)=0\)

\(\Rightarrow x+2004=0\)

\(\Rightarrow x=-2004\)

\(\left(x_1+x_2+x_3\right)+\left(x_4+x_5+x_6\right)+...+\left(x_{1999}+x_{2000}+x_{2001}\right)+x_{2002}=0\)

\(\Rightarrow1+1+1+...+1+x_{2002}=0\) 
       _____________________
              \(\frac{\left(2001-1+1\right)}{3}\)số 1 

\(667+x_{2002}=0\)

\(x_{2002}=-667\)

a, = (1-2-3+4)+(5-6-7+8)+....+(2001-2002-2003+2004) = 0+0+...+0 = 0

b, => x-1=0 hoặc x-10=0 hoặc x=0

=> x=1 hoặc x=10 hoặc x=0

c, => 9x=189

=> x=189:9 = 21

k mk nha

Sao cậu cho mình mà ko biết đó là gì? Bà bán hàng bọc lại rồi đưa cho cậu à. Cảm ơn, chào!

a) \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x.\left(x+1\right)}=\frac{667}{668}\)

\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{667}{668}\)

\(1-\frac{1}{x+1}=\frac{667}{668}\)

\(\frac{1}{x+1}=1-\frac{667}{668}\)

\(\frac{1}{x+1}=\frac{1}{668}\)

\(\Rightarrow x+1=668\)

x = 667

a) 1/1x2 + 1/2x3 + 1/3x4 + ... + 1/x.(x+1) = 667/668

=>1/1-1/2+1/2-1/3+1/3-1/4+.......+1/x-1/x+1=667/668

=>1/1-1/x+1=667/668

=>1/x+1=1/1-667/668

=>1/x+1=1/668

=>x=667