1. Tính nhanh a. -1/24 -[1/4 – ( 1/2 – 7/8 ) ] b. ( -3/4 + 5/13 ) : 2/7 – ( 2 1/4 + 8/13 ) :2/7

a) $-\dfrac{1}{24}-[\dfrac{1}{4}-(\dfrac{1}{2}-\dfrac{7}{8})]$

$=-\dfrac{1}{24}-\dfrac{6}{24}+\dfrac{12}{24}-\dfrac{21}{24}$

$=\dfrac{-1-6+12-21}{24}=\dfrac{-16}{24}=\dfrac{-2}{3}$

b) $(-\dfrac{3}{4}+\dfrac{5}{13}):\dfrac{2}{7}-(2\dfrac{1}{4}+\dfrac{8}{13}):\dfrac{2}{7}$

$=(-\dfrac{3}{4}+\dfrac{5}{13}).\dfrac{7}{2}-(\dfrac{9}{4}+\dfrac{8}{13}).\dfrac{7}{2}$

$=\dfrac{7}{2}.(-\dfrac{3}{4}+\dfrac{5}{13}-\dfrac{9}{4}-\dfrac{8}{13})$

$=\dfrac{7}{2}.[(-\dfrac{3}{4}-\dfrac{9}{4})+(\dfrac{5}{13}-\dfrac{8}{13})]$

$=\dfrac{7}{2}.[-3-\dfrac{3}{13}]$

$=\dfrac{7}{2}.(-\dfrac{42}{13})$

$=-\dfrac{147}{13}$

Lưu ý: Có khả năng không thể tính nhanh vì tính nhanh thường là để sẽ ra các chữ số thuộc số nguyên