Tìm x biết 56-x=9
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Ta có \(\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{2}{x.\left(x+1\right)}=\frac{2}{9}\)
\(\Rightarrow\frac{2}{6.7}+\frac{2}{7.8}+\frac{2}{8.9}+...+\frac{2}{x.\left(x+1\right)}=\frac{2}{9}\)
\(2\left(\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{x.\left(x+1\right)}\right)=\frac{2}{9}\)
\(2\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2}{9}\)
\(2\left(\frac{1}{6}-\frac{1}{x+1}\right)=\frac{2}{9}\)
\(\frac{1}{6}-\frac{1}{x+1}=\frac{2}{9}\div2\)
\(\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}\)
\(\frac{1}{x+1}=\frac{1}{6}-\frac{1}{9}\)
\(\frac{1}{x+1}=\frac{1}{18}\left(1\right)\)
Có \(\left(1\right)\Leftrightarrow\left(x+1\right).1=1.18\)
\(\Rightarrow x+1=18\)
\(\Rightarrow x=18-1\)
\(\Rightarrow x=17\)
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\(\frac{1}{6x7}+\frac{1}{7x8}+\frac{1}{8x9}+...+\frac{1}{Xx\left(X+1\right)}=\frac{1}{9}\)
\(\frac{7-6}{6x7}+\frac{8-7}{7x8}+\frac{9-8}{8x9}+...+\frac{\left(X+1\right)-X}{Xx\left(X+1\right)}=\frac{1}{9}\)
\(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{X}-\frac{1}{X+1}=\frac{1}{9}\)
\(\frac{1}{6}-\frac{1}{X+1}=\frac{1}{9}\)
\(\frac{1}{X+1}=\frac{1}{6}-\frac{1}{9}=\frac{1}{18}\)
X+1=18
X=17
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`- 9 . ( x - 6 ) + 20 = 56`
`=> -9 . ( x - 6 ) = 56-20`
`=> -9 . ( x - 6 ) =36`
`=>x-6=36:(-9)`
`=>x-6=-4`
`=>x=-4+6`
`=>x=2`
`-------------`
` ( 245 - x ) + 7^2=149`
`=>( 245 - x ) +49=149`
`=> 245 - x =149-49`
`=> 245 - x =100`
`=>x=245-100`
`=>x=145`
`------------`
`(2^x-3).7=35`
`=> 2^x-3=35:7`
`=> 2^x-3=5`
`=>2^x=5+3`
`=>2^x=8`
`=>2^x=2^3`
`=>x=3`
56-x=9
x=56-9
x=47
vậy x=47
x=56-9
x=47