a) 5/15 + 14/25 – 12/9 + 2/7 + 11/25 =? b) 15 × 1/5 : ( -5/7) – 2 trên 1/5 × ( -7/5 )=?

Đáp án:

a. \(\dfrac{2}{7}\)

Giải thích các bước giải:

\(\begin{array}{l}
a.\dfrac{5}{{15}} + \dfrac{{14}}{{25}} – \dfrac{{12}}{9} + \dfrac{2}{7} + \dfrac{{11}}{{25}}\\
 = \dfrac{1}{3} + \dfrac{{14}}{{25}} – \dfrac{4}{3} + \dfrac{2}{7} + \dfrac{{11}}{{25}}\\
 = \left( {\dfrac{1}{3} – \dfrac{4}{3}} \right) + \left( {\dfrac{{14}}{{25}} + \dfrac{{11}}{{25}}} \right) + \dfrac{2}{7}\\
 =  – 1 + 1 + \dfrac{2}{7} = \dfrac{2}{7}\\
b.\dfrac{{15.\dfrac{1}{5}:\left( { – \dfrac{5}{7}} \right) – 2}}{{\dfrac{1}{5}.\left( { – \dfrac{7}{5}} \right)}}\\
 = \dfrac{{3.\left( { – \dfrac{7}{5}} \right) – 2}}{{\dfrac{1}{5}.\left( { – \dfrac{7}{5}} \right)}}\\
 = \left( { – \dfrac{{21}}{5} – 2} \right):\left( { – \dfrac{7}{{25}}} \right)\\
 =  – \dfrac{{31}}{5}.\left( { – \dfrac{{25}}{7}} \right) = \dfrac{{31.5}}{7} = \dfrac{{155}}{7}
\end{array}\)