${a,}$ $\dfrac{5}{15}+$ $\dfrac{14}{25}-$ $\dfrac{12}{9}+$ $\dfrac{2}{7}+$ $\dfrac{11}{25}$
${=}$ $\dfrac{1}{3}+$ $\dfrac{14}{25}-$ $\dfrac{4}{3}+$ $\dfrac{2}{7}+$ $\dfrac{11}{25}$
${=}$ $(\dfrac{1}{3}-$ $\dfrac{4}{3})+$ $(\dfrac{14}{25}+$ $\dfrac{11}{25})+$ $\dfrac{2}{7}$
${=-1+1+}$ $\dfrac{2}{7}$
${=}$ $\dfrac{2}{7}$
${b,15}$ x $\dfrac{1}{5}:$ $(-\dfrac{5}{7})-$ $\dfrac{2}{15}$ x $(-\dfrac{7}{5})$
${=}$ $-\dfrac{21}{5}-$ $(-\dfrac{14}{75})$
$=-\dfrac{315}{75}-$ $(-\dfrac{14}{75})$
${=}$ $-\dfrac{301}{75}$
~Xin hay nhất~
