Bài 5 trang 27 toán lớp 7 tập 1 CTSTTìm x, biết:a) $\frac{-3}{5}.x=\frac{12}{25}$b) $\frac{3}{5}.x...

Cách làm và câu trả lời chi tiết:

a)
$-\frac{3}{5} \cdot x = \frac{12}{25}$
$x = \frac{12}{25} : (-\frac{3}{5})$
$x = -4$

b)
$\frac{3}{5} \cdot x - \frac{3}{4} = -\frac{3}{2}$
$\frac{3}{5} \cdot x = -\frac{3}{2} + \frac{3}{4}$
$\frac{3}{5} \cdot x = -\frac{3}{4}$
$x = -\frac{3}{4} : \frac{3}{5}$
$x = -\frac{5}{4}$

c)
$\frac{3}{5} : x = \frac{1}{2} - \frac{2}{5}$
$\frac{3}{5} : x = \frac{3}{10}$
$x = \frac{3}{5} : \frac{3}{10}$
$x = \frac{6}{10}$
$x = \frac{3}{5}$

d)
$\frac{3}{4} - (x - \frac{1}{2}) = \frac{5}{3}$
$x - \frac{1}{2} = \frac{3}{4} - \frac{5}{3}$
$x - \frac{1}{2} = -\frac{11}{12}$
$x = -\frac{11}{12} + \frac{1}{2}$
$x = -\frac{5}{12}$

e)
$2\frac{2}{15} : (\frac{1}{3} - 5x) = -2\frac{2}{5}$
$(\frac{1}{3} - 5x) = \frac{32}{15} + \frac{12}{5}$
$(\frac{1}{3} - 5x) = \frac{68}{15}$
$5x = \frac{1}{3} - \frac{68}{15}$
$5x = \frac{5}{15} - \frac{68}{15}$
$5x = -\frac{63}{15}$
$x = \frac{-63}{15} : 5$
$x = -\frac{63}{75}$
$x = -\frac{21}{25}$

f)
$x^2 + \frac{1}{9} = \frac{5}{3} : 3$
$x^2 + \frac{1}{9} = \frac{5}{9}$
$x^2 = \frac{5}{9} - \frac{1}{9}$
$x^2 = \frac{4}{9}$
$x = \pm \frac{2}{3}$

Vậy câu trả lời là: $x = -4; x = -\frac{5}{4}; x = \frac{3}{5}; x = -\frac{5}{12}; x = -\frac{21}{25}; x = \pm \frac{2}{3}$