Câu i) B= 2/9+-10/7+26/35+8/45

Mn giúp mik với

Đáp án:

$\begin{array}{l}
a)\dfrac{7}{{ - 25}} + \dfrac{{ - 8}}{{25}} = \dfrac{{ - 7 - 8}}{{25}} = \dfrac{{ - 15}}{{25}} = \dfrac{{ - 3}}{5}\\
b)\dfrac{{ - 18}}{{24}} - \dfrac{{15}}{{21}}\\
 = \dfrac{{ - 3}}{4} - \dfrac{5}{7}\\
 = \dfrac{{ - 21 - 20}}{{28}}\\
 = \dfrac{{ - 41}}{{28}}\\
c)\dfrac{{ - 3}}{{17}} + \left( {\dfrac{2}{3} + \dfrac{3}{{17}}} \right)\\
 = \dfrac{{ - 3}}{{17}} + \dfrac{3}{{17}} + \dfrac{2}{3}\\
 = \dfrac{2}{3}\\
d)\dfrac{3}{7} + \dfrac{{ - 8}}{9} + \dfrac{{ - 7}}{8}\\
 = \dfrac{{3.72 - 8.56 - 7.63}}{{504}}\\
 = \dfrac{{ - 673}}{{504}}\\
e)\dfrac{{ - 1}}{2} - \dfrac{2}{3} + \dfrac{{ - 3}}{4} + \dfrac{2}{3}\\
 =  - \dfrac{1}{2} - \dfrac{3}{4} - \dfrac{2}{3} + \dfrac{2}{3}\\
 = \dfrac{{ - 5}}{4}\\
g)\dfrac{{ - 4}}{3} - \dfrac{3}{8} + \dfrac{7}{3} - \dfrac{3}{2}\\
 = \left( {\dfrac{{ - 4}}{3} + \dfrac{7}{3}} \right) - \dfrac{3}{8} - \dfrac{3}{2}\\
 = 1 - \dfrac{3}{8} - \dfrac{3}{2}\\
 = \dfrac{{ - 7}}{8}\\
h)A = \dfrac{8}{{11}} + \dfrac{5}{7} - \dfrac{3}{{11}} + \dfrac{2}{7} + \dfrac{6}{{11}}\\
 = \left( {\dfrac{8}{{11}} - \dfrac{3}{{11}} + \dfrac{6}{{11}}} \right) + \left( {\dfrac{5}{7} + \dfrac{2}{7}} \right)\\
 = \dfrac{{11}}{{11}} + \dfrac{7}{7}\\
 = 2\\
i)\\
B = \dfrac{2}{9} + \dfrac{{ - 10}}{7} + \dfrac{{26}}{{35}} + \dfrac{8}{{45}}\\
 = \dfrac{{ - 50 + 26}}{{35}} + \dfrac{2}{5}\\
 = \dfrac{{ - 24}}{{35}} + \dfrac{2}{5}\\
 = \dfrac{2}{7}\\
k)\\
C = \dfrac{2}{{3.5}} + \dfrac{2}{{5.7}} + ... + \dfrac{2}{{37.39}}\\
 = \dfrac{1}{3} - \dfrac{1}{5} + \dfrac{1}{5} - \dfrac{1}{7} + ... + \dfrac{1}{{37}} - \dfrac{1}{{39}}\\
 = \dfrac{1}{3} - \dfrac{1}{{39}}\\
 = \dfrac{{12}}{{39}}\\
 = \dfrac{4}{{13}}
\end{array}$