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 \(x = \frac{{391}}{{56}}.\)