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![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
B. ( x - 21.13 ) : 11 = 30
=> x = 30 x 11 + 21 x 13
=> x = 330 + 273
=> x = 603
C. ( 25 - x ) + ( 29 - x ) + ( 33 - x ) + ... + ( 101 - x ) = 128
=> ( 25 + 29 + 33 + ... + 101 ) - ( x + x + x +...+ x ) = 128
=> 1260 - 20x = 128
=> 20x = 1260 - 128
=> x = 1132 : 20
=> x = 283/5
B. ( x - 21.13 ) : 11 = 30
=> x = 30 x 11 + 21 x 13
=> x = 330 + 273
=> x = 603
C. ( 25 - x ) + ( 29 - x ) + ( 33 - x ) + ... + ( 101 - x ) = 128
=> ( 25 + 29 + 33 + ... + 101 ) - ( x + x + x +...+ x ) = 128
=> 1260 - 20x = 128
=> 20x = 1260 - 128
=> x = 1132 : 20
=> x = 283/5
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{1}{2}B=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{15}-\frac{1}{16}=\frac{1}{4}-\frac{1}{16}=\frac{3}{16}\)
=>\(B=\frac{3}{16}:\frac{1}{2}=\frac{3}{8}\)
\(C=\left(\frac{3}{29}-\frac{1}{5}\right)\cdot\frac{29}{3}=1-\frac{1}{5}\cdot\frac{29}{3}=1-\frac{29}{15}=-\frac{14}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-...-\frac{1}{120}=\frac{5}{8}\)
\(\Rightarrow\frac{x}{2008}-[2\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{240}\right)]=\frac{5}{8}\)
\(\Rightarrow\frac{x}{2008}-[2\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\right)]=\frac{5}{8}\)
\(\Rightarrow\frac{x}{2008}-[2\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{5}-\frac{1}{16}\right)]=\frac{5}{8}\)
\(\Rightarrow\frac{x}{2008}-[2.\frac{3}{16}]=\frac{5}{8}\)
\(\Rightarrow\frac{x}{2008}=1\)
\(\Rightarrow x=2008\)
\(\frac{7}{x}+\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+...+\frac{4}{41.45}=\frac{29}{45}\)
\(\Rightarrow\frac{7}{x}+\left(\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+...+\frac{4}{41.45}\right)=\frac{29}{45}\)
\(\Rightarrow\frac{7}{x}+\left(\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{41}-\frac{1}{45}\right)=\frac{29}{45}\)
\(\Rightarrow\frac{7}{x}+\left(\frac{1}{5}-\frac{1}{45}\right)=\frac{29}{45}\)
\(\Rightarrow\frac{7}{x}+\frac{8}{45}=\frac{29}{45}\)
\(\Rightarrow\frac{7}{x}=\frac{21}{45}\)
\(\Rightarrow x=15\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Bạn kiểm tra lại giùm mình đi nha:
15.37.4+120.21+21.5.12 16.64+76.34
=15.37.4+(6.7).4.(5.3)+(3.5).(7.3).4 =
=15.37.4+42.4.15+15.21.4
=15.4.(37+42+21)
=60.100
=6000
= (-139)
29+21.(120-128)
=29+21.(-8)
=29+(-168)
=-139