Bài1: tìm x , biết

a, 2/7 . X - 3/4 =1/8

b, 6/5 - 1/5 . X = 2/3

c, 1/4 . X - 3/7 = 4/3

d, 6/7 + x . 3/4 = 5/2

Đáp án:

`a)` `\frac{2}{7} \cdot x - \frac{3}{4} = \frac{1}{8}`
`\frac{2}{7} \cdot x = \frac{1}{8} + \frac{3}{4}`
`\frac{2}{7} \cdot x = \frac{1}{8} + \frac{6}{8}`
`\frac{2}{7} \cdot x = \frac{7}{8}`
`x = \frac{7}{8} \cdot \frac{7}{2}`
`x = \frac{49}{16}`
`b)` `frac{6}{5} - \frac{1}{5} \cdot x = \frac{2}{3}`
`- \frac{1}{5} \cdot x = \frac{2}{3} - \frac{6}{5}`
`- \frac{1}{5} \cdot x = \frac{10}{15} - \frac{18}{15}`
`- \frac{1}{5} \cdot x = - \frac{8}{15}`
`x = - \frac{8}{15} \cdot (-5)`
`x = \frac{40}{15}`
`x = \frac{8}{3}`
`c)` `\frac{1}{4} \cdot x - \frac{3}{7} = \frac{4}{3}`
`\frac{1}{4} \cdot x = \frac{4}{3} + \frac{3}{7}`
`\frac{1}{4} \cdot x = \frac{28}{21} + \frac{9}{21}`
`\frac{1}{4} \cdot x = \frac{37}{21}`
`x = \frac{37}{21} \cdot 4`
`x = \frac{148}{21}`
`d)` `\frac{6}{7} + x \cdot \frac{3}{4} = \frac{5}{2}`
`x \cdot \frac{3}{4} = \frac{5}{2} - \frac{6}{7}`
`x \cdot \frac{3}{4} = \frac{35}{14} - \frac{12}{14}`
`x \cdot \frac{3}{4} = \frac{23}{14}`
`x = \frac{23}{14} \cdot \frac{4}{3}`
`x = \frac{92}{42}`
`x = \frac{46}{21}`