\(a.\dfrac{12+x}{42}=\dfrac{35}{42}\Leftrightarrow12+x=35\Leftrightarrow x=23\)

\(b.\dfrac{25-x}{40}=\dfrac{3}{8}\Leftrightarrow\dfrac{25-x}{40}=\dfrac{15}{40}\Leftrightarrow25-x=15\Leftrightarrow x=10\)

\(c.\dfrac{13+x}{20}=\dfrac{3}{4}\Leftrightarrow\dfrac{13+x}{20}=\dfrac{15}{20}\Leftrightarrow13+x=15\Leftrightarrow x=2\)

\(d.\dfrac{23-x}{25}=\dfrac{20}{25}\Leftrightarrow23-x=20\Leftrightarrow x=3\)

1/2 + 1/6 + 1/12 + 1/20 + 1/30 1/x = 41/42

5/6 + 1/x = 41/42

1/x = 41/42 - 5/6 

1/x = 1/7 

vậy x = 7

x = 7 ở vong 18 đúng không

chuẩn 

Ghi rõ đề ra, VP là bao nhiêu?

Biến đổi VT:

\(x+\frac{x}{2}+\frac{x}{6}+\frac{x}{12}+\frac{x}{20}+\frac{x}{30}+\frac{x}{42}+\frac{x}{56}\)

\(=x\left(1+\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{6.7}+\frac{1}{7.8}\right)\)

\(=x\left(1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\right)\)

\(=x\left(2-\frac{1}{8}\right)\)

\(=x.\frac{15}{8}\)

\(x+\frac{x}{2}+\frac{x}{6}+\frac{x}{12}+\frac{x}{20}+\frac{x}{30}+\frac{x}{42}+\frac{x}{56}=28\)

\(\Leftrightarrow x\left(1+\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\right)=28\)

\(\Leftrightarrow x\left(1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}\right)=28\)

\(\Leftrightarrow x\left(1+1-\frac{1}{8}\right)=28\)

\(\Leftrightarrow x.\frac{15}{8}=28\)

\(\Leftrightarrow x=\frac{224}{15}\)

a) 12 : x = 3

     X = 12 : 3

     X = 4

b) 21 : x = 7

     X = 21 : 7

     X = 3

c) 30 : x = 3

     X = 30 : 3

     X = 10

d) X : 7 = 4

     X = 4 x 7

     X = 28

e) 20 : x = 5

     X = 20 : 5

     X = 4

f) X x 6 = 42

     X = 42 : 6

     X = 7

b: \(\Leftrightarrow25-x=30\)

hay x=-5