B= $\frac{5}{13}$.$\frac{8}{15}$+$\frac{5}{13}$.$\frac{26}{15}$-$\frac{5}{13}$.$\frac{8}{15}$
B=$\frac{5}{13}$.($\frac{8}{15}$+$\frac{26}{15}$-$\frac{8}{15}$)
B=$\frac{5}{13}$.$\frac{26}{15}$
B=$\frac{2}{3}$.
6.39: Tính một cách hợp lí .
B= $\frac{5}{13}$.$\frac{8}{15}$+$\frac{5}{13}$.$\frac{26}{15}$-$\frac{5}{13}$.$\frac{8}{15}$.
B= $\frac{5}{13}$.$\frac{8}{15}$+$\frac{5}{13}$.$\frac{26}{15}$-$\frac{5}{13}$.$\frac{8}{15}$
B=$\frac{5}{13}$.($\frac{8}{15}$+$\frac{26}{15}$-$\frac{8}{15}$)
B=$\frac{5}{13}$.$\frac{26}{15}$
B=$\frac{2}{3}$.