Question

Tính một cách hợp lí:

a) \(\dfrac{3}{4}.\dfrac{1}{{13}} - \dfrac{3}{4}.\dfrac{{14}}{{13}}\)

b) \(\dfrac{5}{{13}}.\dfrac{{ - 3}}{{10}}.\dfrac{{ - 13}}{5}\)

Answer 1

a) \(\dfrac{3}{4}.\dfrac{1}{{13}} - \dfrac{3}{4}.\dfrac{{14}}{{13}}\)

\(\begin{array}{l} = \dfrac{3}{4}.\left( {\dfrac{1}{{13}} - \dfrac{{14}}{{13}}} \right) = \dfrac{3}{4}.\left( {\dfrac{{1 - 14}}{{13}}} \right)\\ = \dfrac{3}{4}.\dfrac{{-13}}{{13}} = \dfrac{3}{4}.(-1 )= \dfrac{-3}{4}\end{array}\)

b) \(\dfrac{5}{{13}}.\dfrac{{ - 3}}{{10}}.\dfrac{{ - 13}}{5}\)

\(\begin{array}{l} = \dfrac{5}{{13}}.\dfrac{{ - 13}}{5}.\dfrac{{ - 3}}{{10}} = \left( {\dfrac{5}{{13}}.\dfrac{{ - 13}}{5}} \right).\dfrac{{ - 3}}{{10}}\\ = \dfrac{{5.\left( { - 13} \right)}}{{13.5}}.\dfrac{{ - 3}}{{10}} = \left( { - 1} \right).\dfrac{{ - 3}}{{10}} = \dfrac{3}{{10}}\end{array}\)