\(a,\dfrac{5}{12}+\dfrac{-5}{7}+\dfrac{-20}{41}+\dfrac{7}{12}+\dfrac{-21}{41}\)
\(=\left(\dfrac{5}{12}+\dfrac{7}{12}\right)-\left(\dfrac{20}{41}+\dfrac{21}{41}\right)+\dfrac{-5}{7}\)
\(=1-1+\dfrac{-5}{7}\)
\(=\dfrac{-5}{7}\)
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\(b,\dfrac{-1}{3}-\dfrac{2}{21}=\dfrac{-7}{21}-\dfrac{2}{21}=\dfrac{-9}{21}\)
`a)5/12+(-5/7)+(-20/41)+7/12+(-21/41)`
`=(5/7+7/12)+(-5/7)+[(-20/41)+(-21/41)]`
`=12/12+(-5/7)+(-41/41)`
`=1+(-5/7)+(-1)`
`=(-1+1)+(-5/7)`
`=0+(-5/7)`
`=-5/7`
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`b)` Hiệu là :
`-1/3-2/21=-9/21`
( 5/12 + 7/12 ) + ( -21/41 + -20/41 ) + ( -5/7 )
\(\Leftrightarrow\) 1 -1 + (-5/7 )
\(=\) -5/7
