(2 f,=2×2i+(19-2×2)j=4i+11 -2i-40 dt 可=2i-87 (ms) ,=2+(89=825m-s =号-7xr i j i j t 2 2 19 2 2 4 11 2 2 1 2 2 8 v i j ms t i tj dt dr v 2 4 2 2 1 2 2 8 8.25 v ms t 75 58 2 1 8 t g (2)