Tính một cách hợp lí:
\(\begin{array}{l}a)A = \frac{{ - 5}}{7} + \frac{8}{{11}} + ( - \frac{2}{7}) + \frac{1}{2} + \frac{3}{{11}};\\b)B = \frac{{11}}{{17}} + ( - \frac{8}{{19}}) + ( - \frac{3}{4}) + \frac{6}{{17}} - \frac{{30}}{{19}}.\end{array}\)
Nhóm các phân số có cùng mẫu số với nhau
\(\begin{array}{l}a)A = \frac{{ - 5}}{7} + \frac{8}{{11}} + ( - \frac{2}{7}) + \frac{1}{2} + \frac{3}{{11}}\\ = [\frac{{ - 5}}{7} + ( - \frac{2}{7})] + (\frac{8}{{11}} + \frac{3}{{11}}) + \frac{1}{2}\\ = \frac{{ - 7}}{7} + \frac{{11}}{{11}} + \frac{1}{2}\\ = ( - 1) + 1 + \frac{1}{2}\\ = \frac{1}{2}\\b)B = \frac{{11}}{{17}} + ( - \frac{8}{{19}}) + ( - \frac{3}{4}) + \frac{6}{{17}} - \frac{{30}}{{19}}.\\ = (\frac{{11}}{{17}} + \frac{6}{{17}}) + [( - \frac{8}{{19}}) - \frac{{30}}{{19}}] + ( - \frac{3}{4})\\ = \frac{{17}}{{17}} + \frac{{ - 38}}{{19}} + ( - \frac{3}{4})\\ = 1 + ( - 2) + ( - \frac{3}{4})\\ = \frac{{ - 7}}{4}\end{array}\)