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 coordinateof the center of mass.
As to the conservation of motion ofcenterofmass, 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。
3
And then using masscenter formula to calculate four parameters of translation axes and rotation coordinate, mathematic model is established.