1. The problem statement, all variables and given/known data I have three particles of mass m at (-a,-a), (a,-a) and (0,a) Find the moment of inertia I_z around the center of mass of the system for an axis along the z-axis. 2. Relevant equations Center of mass: CM = Ʃmr Moment of inertia: I = Ʃmp^2 m = mass r = position p = distance from axis 3. The attempt at a solution I found that the center of mass is CM = (0,-a/3); CM_x = (-am + am + 0m)/3m = 0 CM_y = (-a-a+a)m/3m = -a/3 This is something I know is correct.. The real problem starts with the moment of inertia: p_1 = 4a/3 p_2 = p_3 = √(a^2 + (2a/3)^2) (p_1)^2 = (16/9)a^2 (p_2)^2 = (a^2 + (2a/3)^2) = (13/9)a^2 Ʃmp^2 = m(16/9)a^2 + m(13/9)a^2 + m(13/9)a^2 = m((16/9)a^2 + (26/9)a^2) ≈ 4.67ma^2 The problem is that the book says that the solution is 4.01ma^2. Where have I made a mistake?