Đáp án:
a) 3/7
Giải thích các bước giải:
\[\begin{array}{l}a)\frac{{15}}{{34}} + \frac{7}{{21}} + \frac{{19}}{{34}} - \frac{{20}}{{15}} + \frac{3}{7}\\ = \left( {\frac{{15}}{{34}} + \frac{{19}}{{34}}} \right) + \left( {\frac{7}{{21}} + \frac{3}{7}} \right) - \frac{{20}}{{15}}\\ = \,\,\,\,\,\,\,\,\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\, + \,\,\,\left( {\frac{1}{3} + \frac{3}{7}} \right) - \frac{4}{3}\\ = \,\,\,\,\,\,\,\,\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\, + \left( {\frac{1}{3} - \frac{4}{3}} \right) + \frac{3}{7}\\ = \,\,\,\,\,\,\,\,\,\,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + \left( { - 1} \right)\,\,\,\,\,\,\,\,\, + \frac{3}{7}\\ = \frac{3}{7}\end{array}\]
