a) \(\dfrac{3}{4}\)+\(\dfrac{7}{-12}\)=\(\dfrac{-9}{-12}\)+\(\dfrac{7}{-12}\)=\(\dfrac{-2}{-12}\)=\(\dfrac{1}{6}\)
b)\(\dfrac{5}{9}\)+\(\dfrac{-7}{12}\)-\(\dfrac{-4}{9}\)+\(\dfrac{-5}{12}\)=(\(\dfrac{5}{9}\)-\(\dfrac{-4}{9}\))+(\(\dfrac{-7}{12}\)+\(\dfrac{-5}{12}\))=1+(-1)=0
c)\(\dfrac{2019}{2020}\).(\(\dfrac{1}{3}-\dfrac{7}{12}+\dfrac{1}{4}\))-25%=\(\dfrac{2019}{2020}\).(\(\dfrac{4}{12}-\dfrac{7}{12}+\dfrac{3}{12}\))-\(\dfrac{1}{4}\)=\(\dfrac{2019}{2020}\).0-\(\dfrac{1}{4}\)=\(\dfrac{-1}{4}\)
Giải:
a) 3/4+7/-12=9/12+-7/12=1/6
b)5/9+-7/12--4/9+-5/12
=(5/9--4/9)+(-7/12+-5/12)
=1+(-1)
=0
c)2019/2020.(1/3-7/12+1/4)-25%
=2019/2020.0-1/4
=0-1/4
=-1/4
