a) Tính: \(\frac{7}{{10}} \cdot \frac{{15}}{{19}} + \frac{7}{{10}} \cdot \frac{4}{{19}}\).
b) Tìm x , biết: \(0,8 - \left( {{\rm{x + }}\frac{3}{5}} \right) = \frac{1}{2}\).
a) Nhóm nhân tử chung để tính.
b) Sử dụng quy tắc chuyển vế để tìm x.
a) \(\frac{7}{{10}} \cdot \frac{{15}}{{19}} + \frac{7}{{10}} \cdot \frac{4}{{19}}\)
\(\begin{array}{l} = \frac{7}{{10}}\left( {\frac{{15}}{{19}} + \frac{4}{{19}}} \right)\\ = \frac{7}{{10}}.1\\ = \frac{7}{{10}}\end{array}\)
b) \(0,8 - \left( {{\rm{x + }}\frac{3}{5}} \right) = \frac{1}{2}\)
\(\begin{array}{l}\frac{4}{5} - \left( {{\rm{x + }}\frac{3}{5}} \right) = \frac{1}{2}\\x + \frac{3}{5} = \frac{4}{5} - \frac{1}{2}\\x + \frac{3}{5} = \frac{3}{{10}}\\x = \frac{3}{{10}} - \frac{3}{5}\\x = \frac{{ - 3}}{{10}}\end{array}\)
Vậy \(x = \frac{{ - 3}}{{10}}\).