Giải bài 4 trang 16 SGK Toán 7 tập 1 - Cánh diều

Đề bài

Tìm x, biết:

a)$x + \left( { - \frac{1}{5}} \right) = \frac{{ - 4}}{{15}}$;                         

b)$3,7 - x = \frac{7}{{10}};$

c)$x.\frac{3}{2} = 2,4$;                                  

d)$3,2:x =  - \frac{6}{{11}}$.

Lời giải chi tiết

a)

$\begin{array}{l}x + \left( { - \frac{1}{5}} \right) = \frac{{ - 4}}{{15}}\\x = \frac{{ - 4}}{{15}} + \frac{1}{5}\\x = \frac{{ - 4}}{{15}} + \frac{3}{{15}}\\x = \frac{{ - 1}}{{15}}\end{array}$                 

Vậy $x = \frac{{ - 1}}{{15}}$.

b)

$\begin{array}{l}3,7 - x = \frac{7}{{10}}\\x = 3,7 - \frac{7}{{10}}\\x = \frac{{37}}{{10}} - \frac{7}{{10}}\\x=\frac{30}{10}\\x = 3\end{array}$

Vậy $x = 3$.

c)

$\begin{array}{l}x.\frac{3}{2} = 2,4\\x.\frac{3}{2} = \frac{{12}}{5}\\x = \frac{{12}}{5}:\frac{3}{2}\\x = \frac{{12}}{5}.\frac{2}{3}\\x = \frac{8}{5}\end{array}$

Vậy $x = \frac{8}{5}$       

d)

$\begin{array}{l}3,2:x =  - \frac{6}{{11}}\\\frac{{16}}{5}:x =  - \frac{6}{{11}}\\x = \frac{{16}}{5}:\left( { - \frac{6}{{11}}} \right)\\x = \frac{{16}}{5}.\frac{{ - 11}}{6}\\x = \frac{{ - 88}}{{15}}\end{array}$

Vậy $x = \frac{{ - 88}}{{15}}$.