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下列计算正确的是 ( )
3 a 2 + 4 a 2 = 7 a 4
a 2 ⋅ 1 a = 1
− 18 + 12 ÷ ( − 3 2 ) = 4
a 2 a − 1 − a − 1 = 1 a − 1
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