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Câu `12.`
$\bullet$ `a^2`
`(-1)^2=1`
`( (-1)/2)^2 = 1/4`
`( (-1)/3)^2 = 1/9`
`0^2 = 0`
`(2/3)^2 = 4/9`
$\bullet$ `a^3`
`(-1)^3=-1`
`( (-1)/2)^3=(-1)/8`
`( (-1)/3)^3=(-1)/27`
`0^3=0`
`(2/3)^3=8/27`
$\bullet$ `a^4`
`(-1)^4=1`
`( (-1)/2)^4=1/16`
`( (-1)/3)^4 = 1/81`
`0^4=0`
`(2/3)^4=16/81`
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Câu `13.`
`( (-2)/5)^3 = ( (-2)/5) . ( (-2)/5) . ( (-2)/5) = (-8)/125`
`( (-3)/7)^2 = ( (-3)/7) . ( (-3)/7)=9/49`
`[( (-1)/3)^2]^3= ( (-1)/3)^6 = 1/729`