p=∑(m,D:) AB m vci +m2c2 B 3 777777777 +m3vc3 解： P =mvcix +mvc2x+mvc3x P,mvciy+mvc2y mvcsy3 3 2 2 1 1 C C C m v m v m v    + + = 解： Px = mvC1x + mvC2x + mvC3x Py = m vC1y + m vC2 y + m vC3y