下列计算中，正确的有（      ）
①$a^m\times a^n=a^{mn}$；②${\left(a^{m+n}\right)}^2=a^{2m+n}$；
③$\left(2a^n{b}^3\right)\times \left(-\frac{1}{6}a{b}^{n-1}\right)=-\frac{1}{3}{a}^{n+1}{b}^{n+2}$；④$a^6÷a^3=a^2$。

判断对错．

两个完全一样的梯形一定能拼成一个平行四边形．

有50件产品编号从1到50,现在从中抽取5件检验,用系统抽样确定所抽取的编号为(    )

删除：把多余的词用斜线（\）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1）每处错误及其修改均仅限一词；

2）只允许修改10处，多者（从第11处起）不计分。

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