Đáp án:
\(\begin{array}{l}
a. - \dfrac{1}{{12}}\\
b. - 1
\end{array}\)
Giải thích các bước giải:
\(\begin{array}{l}
a. - \dfrac{1}{{21}} - \dfrac{1}{{28}} = \dfrac{{ - 28 - 21}}{{21.28}} = - \dfrac{{49}}{{588}} = - \dfrac{1}{{12}}\\
b. - \dfrac{8}{{18}} - \dfrac{{15}}{{27}} = - \dfrac{4}{9} - \dfrac{5}{9}\\
= - \dfrac{9}{9} = - 1\\
c.\dfrac{3}{7} - \dfrac{5}{2} - \dfrac{3}{5} = \dfrac{{3.2.5 - 5.5.7 - 3.2.7}}{{7.2.5}}\\
= \dfrac{{30 - 175 - 42}}{{70}} = - \dfrac{{187}}{{70}}\\
d.\dfrac{2}{3} - \left[ {\dfrac{{ - 7}}{4} - \left( {\dfrac{1}{2} + \dfrac{3}{8}} \right)} \right] = \dfrac{2}{3} - \left[ {\dfrac{{ - 7}}{4} - \left( {\dfrac{{4 + 3}}{8}} \right)} \right]\\
= \dfrac{2}{3} - \left[ {\dfrac{{ - 7}}{4} - \dfrac{7}{8}} \right]\\
= \dfrac{2}{3} - \left( {\dfrac{{ - 14 - 7}}{8}} \right)\\
= \dfrac{2}{3} - \left( { - \dfrac{{21}}{8}} \right) = \dfrac{2}{3} + \dfrac{{21}}{8}\\
= \dfrac{{16 + 63}}{{24}} = \dfrac{{79}}{{24}}\\
e.\left( {\dfrac{1}{5} - \dfrac{1}{5}} \right) + \left( { - \dfrac{3}{7} + \dfrac{3}{7}} \right) + \left( {\dfrac{5}{9} - \dfrac{5}{9}} \right) + \left( { - \dfrac{2}{{11}} + \dfrac{2}{{11}}} \right) + \left( {\dfrac{7}{{13}} - \dfrac{7}{{13}}} \right) - \dfrac{9}{{16}}\\
= - \dfrac{9}{{16}}\\
f.\dfrac{1}{{100}} - \dfrac{1}{{100.99}} - \dfrac{1}{{99.98}} - ... - \dfrac{1}{{3.2}} - \dfrac{1}{{2.1}}\\
= \dfrac{1}{{100}} - \dfrac{1}{{100}} + \dfrac{1}{{99}} - \dfrac{1}{{99}} + \dfrac{1}{{98}} - ... - \dfrac{1}{3} + \dfrac{1}{2} - \dfrac{1}{2} + 1\\
= 1\\
g.\dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{5} + \dfrac{3}{7} + \dfrac{1}{6} - \dfrac{4}{{35}}\\
= \left( {\dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{6}} \right) + \left( {\dfrac{1}{5} + \dfrac{3}{7} - \dfrac{4}{{35}}} \right)\\
= \left( {\dfrac{{3 + 2 + 1}}{6}} \right) + \left( {\dfrac{{7 + 15 - 4}}{{35}}} \right)\\
= 1 + \dfrac{{18}}{{35}} = \dfrac{{35 + 18}}{{35}} = \dfrac{{53}}{{38}}
\end{array}\)
