It is still,for the the double integral of r squared dA.

转动惯量仍然是r^2dA的二重积分。

And we want to show that this is equal to the double integral of P sub x Q sub y dA.

接下来要证明的是,这是等于dA的二重积分。

To give better advice on teaching, futher discussion, is made on how to simplify the double integral operation with symmetry, precisely and effectively.

为更好地指导教学，文章还对如何准确、有效地利用对称性简化二重积分的计算作了进一步的探讨。