Thực hiện các phép tính (bằng cách hợp lý nếu có): a) 2/7+(-17/7+8/7) b) 8:(-3/5)-6:(-3/5)

a) $\frac{2}{7}+(\frac{-17}{7}+\frac{8}{7})$     

$=\frac{2}{7}+\frac{-17}{7}+\frac{8}{7}$

$=\frac{2+(-17)+8}{7}$

$=\frac{-7}{7}=-1$.

b) $8:\left(\frac{-3}{5}\right)-6:\left(\frac{-3}{5}\right)$

$=(8-6):\frac{-3}{5}$

$=2:\frac{-3}{5}$

$=2.\frac{-5}{3}=\frac{-10}{3}$.

c) $\frac{2^{15}.9^3}{6^7.4^4}$

$=\frac{2^{15}.3^6}{2^7.3^7.2^8}$

$=\frac{2^{15}.3^6}{2^{15}.3^7}=\frac{1}{3}$.