position vector

(a) If vector r is parralle to vector i, there is no component in vector j,
i.e. 12-t = 0
t = 12

(b) If vector r is parallel to vector (i+j), i.e. the ratio of component of vector i to that of vector j is equal,
i.e. t^2/(12-t) = 1/1 = 1
t^2 = 12 - t
t^2 + t -12 = 0
(t+4)(t-3) = 0
t = 3 or -4

2007-10-28 19:40:17 補充：
t = -4 is rejected because time t >= 0Hence t = 3 only