Tính một cách hợp lí.
\(\begin{array}{l}a)A = - \frac{{11}}{{13}} + \frac{8}{{19}} + ( - \frac{{54}}{{13}}) + \frac{1}{5} + \frac{{11}}{{19}};\\b)B = \frac{{17}}{{20}} + ( - \frac{8}{{21}}) + ( - \frac{3}{4}) + \frac{{29}}{{21}} - \frac{{37}}{{20}}.\end{array}\)
Nhóm các số hạng có cùng mẫu số với nhau.
\(\begin{array}{l}a)A = - \frac{{11}}{{13}} + \frac{8}{{19}} + ( - \frac{{54}}{{13}}) + \frac{1}{5} + \frac{{11}}{{19}}\\ = [( - \frac{{11}}{{13}}) + ( - \frac{{54}}{{13}})] + (\frac{8}{{19}} + \frac{{11}}{{19}}) + \frac{1}{5}\\ = ( - \frac{{65}}{{13}}) + \frac{{19}}{{19}} + \frac{1}{5}\\ = ( - 5) + 1 + \frac{1}{5}\\ = - \frac{{19}}{5}\\b)B = \frac{{17}}{{20}} + ( - \frac{8}{{21}}) + ( - \frac{3}{4}) + \frac{{29}}{{21}} - \frac{{37}}{{20}}\\ = [( - \frac{8}{{21}}) + \frac{{29}}{{21}}] + (\frac{{17}}{{20}} - \frac{{37}}{{20}}) + ( - \frac{3}{4})\\ = \frac{{21}}{{21}} + ( - \frac{{20}}{{20}}) + ( - \frac{3}{4})\\ = 1 + ( - 1) + ( - \frac{3}{4})\\ = - \frac{3}{4}\end{array}\)