→卩c=-14t-5t-2t=-2lt 而|c|=3 卩·c 故l=Pr元= t 7 故,所求向量为 5 77 v  c = −14t − 5t − 2t   = −21t 而 | c |= 3  故 | | 1 Pr c v c j vc      = = = −7t 7 1  t = − 故，所求向量为       = 7 1 , 7 5 v 1, 