a, |x - 3| - 5 = 7x

=> |x - 3| = 7x + 5

Đk: 7x + 5 ≥ 0 => x ≥ -5/7

Ta có:  |x - 3| = 7x + 5

\(\Rightarrow\orbr{\begin{cases}x-3=7x+5\\x-3=-7x-5\end{cases}\Rightarrow}\orbr{\begin{cases}-6x=8\\8x=-2\end{cases}\Rightarrow}\orbr{\begin{cases}x=\frac{-4}{3}\left(ktm\right)\\x=\frac{-1}{4}\left(tm\right)\end{cases}}\Rightarrow x=\frac{-1}{4}\)

b, 209 - |x - 209| = x

=> |x - 209| = 209 - x

Đk: 209 - x ≥ 0 => x ≤ 209

Ta có: |x - 209| = 209 - x

\(\Rightarrow\orbr{\begin{cases}x-209=209-x\\x-209=x-209\end{cases}\Rightarrow}\orbr{\begin{cases}2x=418\\0x=0\forall x\le209\end{cases}\Rightarrow\orbr{\begin{cases}x=209\\x\le209\end{cases}}}\)

=> x ≤ 209

c, (x - 1)²⁰⁰⁸ + (y - 1)²⁰⁰⁸ + |x + y + z| = 0

Vì (x - 1)²⁰⁰⁸ ≥ 0 ; (y - 1)²⁰⁰⁸  ≥ 0 ; |x + y + z| ≥ 0

=> (x - 1)²⁰⁰⁸ + (y - 1)²⁰⁰⁸ + |x + y + z| ≥ 0

Dấu " = " xảy ra <=> \(\hept{\begin{cases}\left(x-1\right)^{2008}=0\\\left(y-1\right)^{2008}=0\\\left|x+y+z\right|=0\end{cases}\Leftrightarrow}\hept{\begin{cases}x-1=0\\y-1=0\\x+y+z=0\end{cases}\Leftrightarrow}\hept{\begin{cases}x=1\\y=1\\1+1+z=0\end{cases}\Leftrightarrow\hept{\begin{cases}x=y=1\\z=-2\end{cases}}}\)

a)=> (2008+x).2008/2=2008

=>(2008+x)=2

=>x=-2006

b, \(\frac{x+1}{2009}+\frac{x+2}{2009}=\frac{x+10}{2000}+\frac{x+11}{1999}\)

\(\Rightarrow\left(\frac{x+1}{2009}+1\right)+\left(\frac{x+2}{2008}+1\right)=\left(\frac{x+10}{2000}+1\right)+\left(\frac{x+11}{1999}+1\right)\)

\(\Rightarrow\frac{x+1+2009}{2009}+\frac{x+2+2008}{2008}=\frac{x+10+2000}{2000}+\frac{x+11+1999}{1999}\)

\(\Rightarrow\frac{x+2010}{2009}+\frac{x+2010}{2008}=\frac{x+2010}{2000}+\frac{x+2010}{1999}\)

\(\Rightarrow\frac{x+2010}{2009}+\frac{x+2010}{2008}-\frac{x+2010}{2000}-\frac{x+2010}{1999}=0\)

\(\Rightarrow\left(x+2010\right)\left(\frac{1}{2009}+\frac{1}{2008}-\frac{1}{2000}-\frac{1}{1999}\right)=0\)

Mà \(\frac{1}{2009}+\frac{1}{2008}-\frac{1}{2000}-\frac{1}{1999}\ne0\)

=> x + 2010 = 0 => x = -2010

ai la Fc cua lam chan khang kb duoc khong?

câu b )

ta phân phối 2 vô 

=> \(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+\frac{2}{17.19}+\frac{x}{209}=\frac{10}{209}\)

\(\Rightarrow\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{17}-\frac{1}{19}+\frac{x}{209}=\frac{10}{209}\)

\(\Rightarrow\frac{8}{209}+\frac{x}{209}=\frac{10}{209}\)

\(\Rightarrow\frac{x}{209}=\frac{2}{209}\)

\(\Rightarrow x=2\)

240 - (120 : x -15) x 20 = 140

        (120: x - 15)  x 20   =240-140

                (120: x - 15 ) = 100:20

                 (120:x-15)=5 

                   120:x=5+15

                    120:x=20

                     x=120:20=6

tíck cho mình thì mình làm nốt bài kia

a) đề lag ak bn?

b)  -50-(25+x)=70

           25+x =-50-70

           25+x =-120

                x =-120-25 =-145