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下列运算正确的是 ( )
3 b 4 a ⋅ 2 a 9 b 2 = b 6
1 3 ab ÷ 2 b 2 3 a = b 3 2
1 2 a + 1 a = 2 3 a
1 a - 1 - 1 a + 1 = 2 a 2 - 1
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