如图所示是使用交流电频率为50Hz的打点计时器测定匀加速直线运动的加速度时得到的一条纸带，相邻两个计数点之间有4个点未画出，测出s₁="1.2cm," s₂="2.4cm," s₃="3.6cm," s₄="4.8cm," 则打下A点时,物体的速度v_A=      cm/s。