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下列计算正确的是 ( )
x 2 + 3 x 2 = 4 x 4
x 2 y · 2 x 3 = 2 x 4 y
( 6 x 3 y 2 ) ÷ ( 3 x ) = 2 x 2
( - 3 x ) 2 = 9 x 2
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