18.Tính hợp lí:
a) (11 + 1/2 - 2/3) - (9 + 5/3 - 3/2) - (2 + 5/2 - 7/3)
b) 5/7 x 5/11 + 5/7 x 2/11 - 5/7 x 14/11
c) (-4/7 + 1/3) x 3/2 + (2/3 - 3/7) : 2/3
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