A car in a roller coaster moves along a track that consists of a sequence of ups and downs. Let the x axis be parallel to the ground and the positive y axis point upward. In the time interval from t=0 to t=4 s, the trajectory of the car along a certain section of the track is given by r? =A(1m/s)ti^+A[(1m/s3)t3?6(1m/s2)t2]j^, where A is a positive dimensionless constant. Derive a general expression for the speed v of the car.