Cathy was born blind(天生是盲的), so she has __________ seen our beautiful world.
计算:abc•c2a2的结果是( )
一列火车总质量m=5×105kg,发动机的额定功率P=6×105W,在轨道上行驶时,轨道对列车的阻力Ff是车重的0.01倍(取g=10m/s2).