Bài 1:Tìm hiện phép tính(tính hợp lý nếu có thể) a)-3/7+(-4/7+3) c)-2/3.7/9+-2/3.1/9-17/3.-2/9 b)15/7:(-18)/14 Bài 2:tính tổng sau: S=3/5.7+3/7.

Đáp án:

$\begin{array}{l}
a) – \frac{3}{7} + \left( { – \frac{4}{7} + 3} \right)\\
 = \frac{{ – 3}}{7} – \frac{4}{7} + 3\\
 = \frac{{ – 3 – 4}}{7} + 3\\
 =  – 1 + 3 = 2\\
c) – \frac{2}{3}.\frac{7}{9} + \left( { – \frac{2}{3}} \right).\frac{1}{9} – \frac{{17}}{3}.\left( { – \frac{2}{9}} \right)\\
 = \frac{{ – 2}}{3}.\frac{7}{9} + \left( { – \frac{2}{3}} \right).\frac{1}{9} + \left( { – \frac{2}{3}} \right).\frac{{ – 17}}{9}\\
 = \left( { – \frac{2}{3}} \right).\left( {\frac{7}{9} + \frac{1}{9} – \frac{{17}}{9}} \right)\\
 = \left( { – \frac{2}{3}} \right).\left( { – 1} \right)\\
 = \frac{2}{3}\\
b)\frac{{15}}{7}:\left( {\frac{{ – 18}}{{14}}} \right)\\
 = \frac{{15}}{7}:\left( {\frac{{ – 9}}{7}} \right)\\
 = \frac{{15}}{7}.\left( {\frac{{ – 7}}{9}} \right)\\
 = \frac{{15}}{9}\\
 = \frac{5}{3}\\
2)S = \frac{3}{{5.7}} + \frac{3}{{7.9}} + … + \frac{3}{{59.61}}\\
 = \frac{3}{2}.\left( {\frac{2}{{5.7}} + \frac{2}{{7.9}} + … + \frac{2}{{59.61}}} \right)\\
 = \frac{3}{2}.\left( {\frac{1}{5} – \frac{1}{7} + \frac{1}{7} – \frac{1}{9} + … + \frac{1}{{59}} – \frac{1}{{61}}} \right)\\
 = \frac{3}{2}.\left( {\frac{1}{5} – \frac{1}{{61}}} \right)\\
 = \frac{3}{2}.\frac{{61 – 5}}{{5.61}}\\
 = \frac{{84}}{{305}}
\end{array}$