Giải bài 2 trang 17 vở thực hành Toán 7

Đề bài

Tìm$x$, biết:

a,$x - \left( {\frac{5}{4} - \frac{7}{5}} \right) = \frac{9}{{20}};$

b,$9 - x = \frac{8}{7} - \left( { - \frac{7}{8}} \right).$

Lời giải chi tiết

a,

$\begin{array}{l}x - \left( {\frac{5}{4} - \frac{7}{5}} \right) = \frac{9}{{20}}\\ \Leftrightarrow x = \frac{9}{{20}} + \left( {\frac{5}{4} - \frac{7}{5}} \right)\\ \Leftrightarrow x = \frac{9}{{20}} + \frac{5}{4} - \frac{7}{5}\\ \Leftrightarrow x = \frac{9}{{20}} + \frac{{25}}{{20}} - \frac{{28}}{{20}}\\ \Leftrightarrow x = \frac{6}{{20}}\\ \Leftrightarrow x = \frac{3}{{10}}.\end{array}$

Vậy $x = \frac{3}{{10}}.$

 

 

b,

$\begin{array}{l}9 - x = \frac{8}{7} - \left( { - \frac{7}{8}} \right)\\ \Leftrightarrow 9 - x = \frac{8}{7} + \frac{7}{8}\\ \Leftrightarrow 9 - x = \frac{{64}}{{56}} + \frac{{49}}{{56}}\\ \Leftrightarrow 9 - x = \frac{{113}}{{56}}\\ \Leftrightarrow x = 9 - \frac{{113}}{{56}}\\ \Leftrightarrow x = \frac{{504}}{{56}} - \frac{{113}}{{56}}\\ \Leftrightarrow x = \frac{{391}}{{56}}\end{array}$

Vậy $x = \frac{{391}}{{56}}.$