Bài 2 : thực hiện phép tính

a, -1/2+7/21-2/6-5/30 b,5/17-9/15+-2/17 +2/-5

Đáp án:

$a,$ $\dfrac{-1}{2}+\dfrac{7}{21}-\dfrac{2}{6}-\dfrac{5}{30}$ $=-\dfrac{2}{3}$

$b,$ $\dfrac{5}{17}-\dfrac{9}{15}+\dfrac{-2}{17}+\dfrac{2}{-5}$ $=-\dfrac{14}{17}$

Giải thích các bước giải:

$a,$ $\dfrac{-1}{2}+\dfrac{7}{21}-\dfrac{2}{6}-\dfrac{5}{30}$

$=-\dfrac{1}{2}+\dfrac{1}{3}-\dfrac{1}{3}-\dfrac{1}{6}$

$=-\dfrac{1}{2}+\left(\dfrac{1}{3}-\dfrac{1}{3}\right)-\dfrac{1}{6}$

$=-\dfrac{1}{2}+0-\dfrac{1}{6}$

$=-\dfrac{1}{2}-\dfrac{1}{6}$

$=\dfrac{-3-1}{6}$

$=\dfrac{-4}{6}$

$=-\dfrac{2}{3}$

$b,$ $\dfrac{5}{17}-\dfrac{9}{15}+\dfrac{-2}{17}+\dfrac{2}{-5}$

$=\dfrac{5}{17}-\dfrac{9}{15}-\dfrac{2}{17}-\dfrac{2}{5}$

$=\left(\dfrac{5}{17}-\dfrac{2}{17}\right)-\dfrac{9}{15}-\dfrac{2}{5}$

$=\dfrac{3}{17}-\dfrac{9}{15}-\dfrac{2}{5}$

$=\dfrac{3}{17}-\dfrac{3}{5}-\dfrac{2}{5}$

$=\dfrac{3}{17}+\dfrac{-5}{5}$

$=\dfrac{15-85}{85}$

$=\dfrac{-70}{85}$

$=-\dfrac{14}{17}$