从理论和实例两个方面，指出了对称区域上二重积分计算中易出现问题的原因，并给出了避免错误的方法。

From the theory and examples, the article points out why in the symmetric block the calculation of the double integral is easy to go wrong and offers some methods to avoid errors.

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那么，怎么计算这个二重积分呢？

Now, how do we compute that double integral?

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至此我已经得到了，用来计算二重积分的所有量。

Now I have everything I need to compute my double integral.

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