[ATTACH=full]2155[/ATTACH] For part (d), the mass passes through the origin when t = 1, 3, 5, and 7. In other words, the odd numbers on the interval [0, 8]. When I replace t with 1, 3, 5, and 7, I get the point (0, 0). Can this be the reason for the textbook answer as stated by David Cohen?