So, switching the area, moving the area to the other side, I'll get double integral of xdA is the area of origin times the x coordinate of the center of mass.

那么，改变一下区域，把这块移到另一侧，我们得到对xdA的双重积分,是原点那的圆面积乘质心的x的坐标。

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As to the conservation of motion of center of mass, the method of absolute coordinate system is generally used in the teaching meterials of theoretic mechanics.

对于质心运动守恒问题，理论力学教材中一般用绝对坐标法进行计算。

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And then using mass center formula to calculate four parameters of translation axes and rotation coordinate, mathematic model is established.

然后采用重心公式计算出坐标平移旋转的4个参数，建立了坐标平移旋转的数学模型。

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