Tính hợp lý a) 3/8 - 6/18 + 9/27 + 2/16 b) 13/26 - 1/3 -1/2 + 7/21

* Hướng dẫn giải

a)

$\begin{array}{l}\frac{3}{8}-\frac{6}{18}+\frac{9}{27}+\frac{2}{16}\\ =\left(\frac{3}{8}+\frac{2}{16}\right)+\left(-\frac{6}{18}+\frac{9}{27}\right)\\ =\left(\frac{6}{16}+\frac{2}{16}\right)+\left(-\frac{18}{54}+\frac{18}{54}\right)\\ =\frac{1}{2}+0=\frac{1}{2}\end{array}$

b)

$\begin{array}{l}\frac{13}{26}-\frac{1}{3}-\frac{1}{2}+\frac{7}{21}\\ =\frac{1}{2}-\frac{1}{3}-\frac{1}{2}+\frac{1}{3}\\ =\left(\frac{1}{2}-\frac{1}{2}\right)+\left(-\frac{1}{3}+\frac{1}{3}\right)\\ =0+0=0\end{array}$