Giải thích các bước giải:
VD5: `5/11 . 21/8 - 3/11 . 21/8 - 13/11 . 21/8`
`= 21/8 . (5.11 - 3/11 - 13/11)`
`= 21/8 . (-1) = (-21)/8`
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VD6: `18/13 . (-7)/9 - 7/9 . 5/13 + 2 . 7/9`
`= 7/9 . ((-18)/13 - 5/13 + 2)`
`= 7/9 . 3/13 = 7/39`
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VD7: `1/15 . 13/4 + 1/15 . 19/4 - 1/2 . 1/15`
`= 1/15 . (13/4 + 19/4 - 1/2)`
`= 1/15 . 15/2 = 1/2`
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VD8: `(-9/13) . 5/17 + 5/13 . (-8/17)`
`= (-5)/13 . 9/5 . 5/17 - 5/13 . 8/17`
`= (-5)/13 . 9/17 - 5/13 . 8/17`
`= (-5)/13 . (9/17 + 8/17)`
`= (-5)/13 . 1 = (-5)/13`
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VD9: `6/23 . 15/11 - 15/11 . 10/23 - 42/23 . 15/11`
`= 15/11 . (6/23 - 10/23 - 42/23)`
`= 15/11 . (-2) = (-30)/11`
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