Tính bằng cách thuận tiện. (2 điểm)

$\frac{5}{8}+\frac{6}{7}+\frac{1}{7}+\frac{3}{8}$                                                  $\frac{56}{11}\times \frac{9}{7}+\frac{9}{7}\times \frac{21}{11}$

\(\frac{5}{8}\,\, + \,\,\frac{6}{7}\,\, + \,\,\frac{1}{7}\,\, + \,\,\frac{3}{8}\)

\( = \left( {\frac{5}{8}\,\, + \,\,\frac{3}{8}} \right)\,\, + \left( {\frac{6}{7}\,\, + \,\,\frac{1}{7}} \right)\,\,\)

\( = \frac{8}{8}\,\, + \frac{7}{7}\)

= 1 + 1

= 2

\(\frac{{56}}{{11}}\,\, \times \,\,\frac{9}{7}\,\, + \,\,\frac{9}{7}\,\, \times \,\,\frac{{21}}{{11}}\)

\( = \frac{9}{7} \times \left( {\frac{{56}}{{11}}\,\,\, + \,\,\,\,\frac{{21}}{{11}}} \right)\)

\( = \frac{9}{7} \times \frac{{77}}{{11}}\)

\( = \frac{9}{{\not 7}} \times \frac{{\not 7 \times \not 11}}{{\not 11}}\)

= 9