cho P= -5/12-(3/4+7/12-2/8) và Q=(7/41-4/9)-(3/19+7/41)+(4/9-16/19)

* Hướng dẫn giải

Đáp án cần chọn là: B

$\begin{array}{l}P=\frac{-5}{12}-\left(\frac{3}{4}+\frac{7}{12}-\frac{2}{8}\right)\\ =\frac{-5}{12}-\frac{3}{4}-\frac{7}{12}+\frac{2}{8}\\ =\left(\frac{-5}{12}-\frac{7}{12}\right)+\left(-\frac{3}{4}\right)+\frac{2}{8}\\ =-1-\frac{3}{4}+\frac{2}{8}\\ =\frac{-3}{2}\end{array}$

$\begin{array}{l}Q=\left(\frac{7}{41}-\frac{4}{9}\right)-\left(\frac{3}{19}+\frac{7}{41}\right)+\left(\frac{4}{9}-\frac{16}{19}\right)\\ =\frac{7}{41}-\frac{4}{9}-\frac{3}{19}-\frac{7}{41}+\frac{4}{9}-\frac{16}{19}\\ =\left(\frac{7}{41}-\frac{7}{41}\right)+\left(-\frac{4}{9}+\frac{4}{9}\right)+\left(-\frac{3}{19}-\frac{16}{19}\right)\\ =0+0-1=-1\end{array}$

Vì  $\frac{-3}{2}<-1$  nên M < N.